I have a 1998 GMC Sierra, 5.7 liter Vortec 4x4. It has 160k miles. I have noticed that it idles at around 500 rpms when its warmed up. Not sure if this is normal. However, I have noticed a bit more vibration from the motor right when i start it up. Also, as I'm accelerating, my rpms will fluctuate. Not so much at higher rpms, but as I gradually accelerate. I am leaning towards the EGR valve, but I really don't want to replace it if that's not the real problem. Any help in this matter would be appreciated.

Check all your vacuum hoses especially at the PCV coming off the valve cover it is often over looked. The PCV fits loose either put a new bushing in the cover or add some black tape to the PCV at least until you figure out that might be the issue. Also your timing should be at TDC or 2 degrees plus or minus. Contrary to popular belief You can use a timing light you can see the timing mark know about where it is and line up the fly wheel mark accordingly.

You will need to figure out if it is fuel or ignition related.
If you have not done a tune-up by replacing plugs and other parts within the past couple of years, start there. Including the fuel filter.
Is the check engine light on ?

See if cleaning the throttle body helps -

TUTORIAL HOW TO clean Vortec Throttle body 5 7L Chevy Gmc

I would look into the coolant temp sensor .
As it sounds to me that the auto enrichener (choke) is letting go too early.

Check that , and let us know what you find !

Thanks!!

check for vacom leak on engin intake

Camshaft To Crankshaft Position Correlation Fault is what p1345 means. if the problem isnt electrical I would imagine the timing chain could be stretched and the engine is out of time somehow

Yea as long as they are both manual transmissions everything will bolt up the same.If you use your old transfer case you will still have push button.

yes that is fine for a stock engine u should have 10 psi for every 1000 rpm and oil grades and weather play a big part on oil pressure

30-40 psi of oil pressure is fine for many vehicles including your GMC. I am assuming that the truck has more than 100,000 miles on it and still has the original engine in it. By rule of thumb you only need 10 psi per 1000 rpms in a typical street driven engine to give adequate oil pressure to keep the bearings from oil starvation. The oil pressure is typically higher when initial start up occurs because it is more viscous due to the temperature of the oil as the engines oil warms up the visconsity thins out and the oil flows more smoothly. The drop in oil pressure is a normal occurance. Above 45 is more than required for a daily driven street engine. I believe that the specs on that engine only list 30- 45 psi.

check throttle body for chocking