I put a 40 pin cat engine in place of 70 pin cat engine and dont know how to get multiplex gages to work such as oil pressure,coolant temp, speedo, tach gages.

Replace the instrument cluster . GM had stepper motor issues 2003 to 2006 . Pin out won't help you , The info for the instrument cluster gauges come's over class 2 serial data buss (network) from the PCM - engine computer . Do you know what GM class 2 data network is ? All gauge info comes over one wire from the PCM .
How to repair GM Instrument Cluster Speedometer Gauge Stepper
Buick Class 2 Operation Diagnosis 1998

Hooking up a Tech 2 scan tool an checking for code's an data to the instrument cluster would be the correct diagnostic procedure . But you wouldn't know that , not being a Chevy technician . Not having factory service repair info. manual .

Speedometer
The IPC displays the vehicle speed based on the information from the PCM. The PCM calculates the vehicle speed from the vehicle speed sensor (VSS) circuit. The IPC receives a class 2 message from the PCM indicating the vehicle speed. The PCM also calculates the vehicle speed based on the wheel reference vehicle speed class 2 message from the EBCM. The PCM uses the vehicle speed information from the EBCM as a backup. The IPC displays the vehicle speed information from the EBCM when the PCM detects a malfunction in the vehicle speed sensor circuit.
The speedometer defaults to 0 km/h (0 mph) if the IPC detects a loss of class 2 communications with both the PCM and the EBCM.

Engine Coolant Temperature (ECT) Gage
The IPC displays the engine coolant temperature on the analog ECT gage as determined by the PCM. The PCM monitors the signal circuit of the engine coolant temperature sensor. The IPC receives a class 2 message from the PCM indicating the engine coolant temperature. The IPC illuminates the check gages indicator when a high engine coolant temperature condition exists. The engine coolant temperature gage defaults to 75°C (160°F) or below when one of the following conditions occur:
• The PCM detects a malfunction in the engine coolant temperature sensor signal circuit.
• The IPC detects a loss of class 2 communication with the PCM.
For additional informatin on the engine cooling system, refer to Cooling System Description and Operation in Engine Cooling.
Engine Oil Pressure (EOP) Gage
The IPC displays the engine oil pressure on the analog EOP gage as determined by the PCM. The PCM monitors the signal circuit of the engine oil pressure sensor. When the engine oil pressure sensor resistance is high, the engine oil pressure is high. When the engine oil pressure sensor resistance is low, the engine oil pressure is low. The IPC receives a class 2 message from the PCM indicating the engine oil pressure. The IPC illuminates the check gages indicator when a low oil pressure condition exists. The engine oil pressure gage defaults to 0 kPa (0 psi) when the following condition occurs:
• The PCM detects a malfunction in the engine oil pressure sensor signal circuit.
• The IPC detects a loss of class 2 communication with the PCM.
Fuel Gage
The IPC displays the fuel level on the analog fuel gage as determined by the PCM. The PCM monitors the signal circuit of the fuel level sensor. When the fuel level sensor resistance is high, the fuel gage indicates full. When the fuel level sensor resistance is low, the fuel gage indicates empty. The IPC receives a class 2 message from the PCM indicating the filtered fuel level percent. The PCM uses 2 filters for the fuel level percent. The slow filter is used during normal operation. The fast filter is used when the engine is just started or when the vehicle is in Park or Neutral. The fast filter allows the fuel gage to reach the current fuel level quickly. The fuel gage defaults to empty when the following conditions occur:
• The PCM detects a malfunction in the fuel level sensor circuit. Refer to Diagnostic System Check - Instrument Cluster .
• The IPC detects a loss of class 2 communications with the PCM.
Reviewing the fuel system description will further explain how the system functions. Refer to:
• Fuel System Description in Engine Controls - 4.3L
• Fuel System Description in Engine Controls - 4.8L, 5.3L, and 6.0L.

Bad coolant temp sensor. This part takes the place of old style choke, and when bad ECM doesn't know engine has warmed up so fuel mix stays too rich.

An old engine will normally show a lower oil pressure, which may be due to worn bearing, a worn oil pump or both.

A new engine with good bearings will normally show a higher pressure at revs than at idle, and a higher pressure when cold. Putting a worn oil pump back in a new engine (never a good idea without a thorough checkup on it) will modify this situation.

Just going on the readings I'd say your situation now is quite normal. You may even be able to change to an oil one grade less viscous, say 10W instead of 20W depending on your winter temps.

Remove the oil pressure sending unit and install a mechanical oil pressure test gage and check the engine's oil pressure, it should be at least 20PSI at idle hot, if it is ok replace the oil pressure sending unit.

ok iif the instrument cluster does not work.. before i give a solution can you tell me if it have atach and temp gage not a temp gage witha idiot light but a actual temp gage.. because if temp gage and fule gage hands are not in their proper place like if yo uhave a 1/4 tank of gas and it reads way past full and the temp gage is gone out of site way past say 240 degrees then it is your speedo head .. have this trouble the other day on a chevy 2002 cavalier.. replaced the head and wahla the car started. it seems the speedo head is tied in with the computer system on these cars.. and if the tach is not working you need not worry if you cannot find a head with a built in tach.. a normal speedo head with out the tach part will also work.. just when you change the head count the pins on the old one and compare it to the new one if they match then just plug it in and car should start..

try replacing the coolant temp sender,here is a diagram for the 2.3 4cylinder engine its located at rear of engine, and if its a 5.0 engine it will be near oil filter.hope this helps.this diagram is for the 2.3 engine.there is also a diagram of the coolant temp sensor dont change this one, you want the sender,

CHECK GAGES light (1) indicates a potential engine failure(low oil pressure, low coolant, coolant overheating) has occurred.
· Check gages and shut down engine. Gages should read as follows: Oil pressure 5 to 10 psi (34 to 69 kPa) at idle; Water Temp gage below 230 degrees (110 C).
· Check engine oil level (Para 2-11) if OIL PRESS gage indicates low reading. Check coolant level (Para 2-11) if Water Temp gage indicates high reading. Notify Unit Maintenance if levels are low.

If CHECK ENGINE light (2) is on, check engine oil pressure gage (3) and water temperature gage (4). If both indicate normal, proceed with mission and then notify Unit Maintenance. If either one does not indicate normal, stop engine immediately and notify Unit Maintenance

at what? depending on engine temp and rpm, normal oil psi should read 30-40 psi at idle and at cruising speed of 1500 to 2000 rpm should read 60-70 psi. most bonnevilles from 1992 to the last year run of 2005 have the 3.8 liter V6 which was updated over the years.

You need to replace the oil pressure sending unit, this is what tells your gauge what the pressure is. For future information if your oil pressure drops again and its a real failed oil pump within a few minutes you will internally damage the engine. I would get a new Oil Pressure Sending Unit. Let me know if you have any questions.... Thanks!

At idle, at operating temp is there any ticking from top end of engine? Are there any knocking noises brom bottom end?
If no noises, change oil and add some Marvel oil to clean out galleys. If that dosent work, change oil pressure sendingf unit. Also, if you have a mechanical oil pressure gauge, hook it up and take a direct reading. Oil pressure (in any vehicle) at idle should NEVER be less than 15psi. Generally in 4.0 jeep engines it's never less than 30, with a high reading at driving rpm of about 40-60! (but, you did not mention what engine you have)