High idle while warming up

Yes.
The spacer could be leaking inside - the spacer should match the gasket for the base of the carb and the intake.

How many rpm's is it running at when you first start it up and it is running on the choke? Does it gradually start to idle back and then start to die r does it do it all of a sudden and dies?

When running with the choke on, it is on the high idle circuit of the carb, as it warms up the idle should slow down and then idle in the 800 rpm range in Park, 650 in Drive. This will be the low idle circuit range. This transition inside the carb will cover an idle air bleed hole in the venturi of the carb and lean out the mixture. If you notice that the carb smells gassy and black rich unburnt gases are coming from the tailpipe, the problem is located within the carb and requires a carb tech to do a rebuild of the carb. You could try a good carb cleaner like Lucas by putting it into the gas tank, in case you just got a bad tank of gas, but that is just a bandaid for a more deeply rooted problem. Get the carb rebuilt?

Check your vacuum hoses. Holley aftermarket may not have as many vacuum ports as OEM QuadraJets, and you may have a vacuum line that isn't plugged into anything...or you may have a vacuum leak around the carb base gasket. Also, does the Holley have an electric or manual choke? Is it connected?

Have you tried using a spray carburetor cleaner? Also, is the fuel filter clean?

Have you tried setting the idle in drive? If you try that, have a helper setting in there with their foot on the brake and parking brake set. I've seen many high mileage vehicles, that wouldn't idle correctly. Other than an engine overhaul, we would set the idle in drive. At least it wouldn't die every time you came to a stop sign or red light. Yeah, when you put it in park or neutral, the idle would go high, but, you don't drive it in park or neutral.
You can get a carb kit for the carb, see if it helps? The carb kit should have all the instructions you need to adjust settings. In the past every carb kit I used had plenty of instructions. I looked at rockauto.com, they had a carb kit for your vehicle. Find the model number on your carb and make sure it matches their carb number.
If your carb is a holley, you can check the holley web site. I haven't been there in a long time, I don't know if they would have instructions for your carb?

Electric fuel pump most likely has too much pressure for the carb. AFAIK, the Holley 4150/60 & 4165/75 should each demand the usual 4.7-5.3 PSI (4.7 @ idle, 5.3 @ max RPMs), so even the Holley red pump would need some restriction from a regulator. If you want this to be adjusted to its best efficiency, you need to have a fuel pressure gauge installed temporarily, if not on the regulator like I have on my Scout II. FWIW, the regulator I have installed is the Mallory 4307M from CenturyPerformance.com, Summitracing.com, Jegs.com, etc.

http://www.malloryperformance.com/pdf/4307M.pdf

A bad Needle & Seat will do this also, or debris stuck between the Needle and the Seat. This is where light tapping on the bowl helps, but it's best to run the fuel out of the bowl a few times and then the volume flow, or quick filling of the bowl, should loosen up the debris.

You also could have set he fuel float too high. What model carb, the ones I mentioned above (AKA Holley Modular Carbs)? If so, then Holley's site has a tech center & instructions on setting the float(s). www.holley.com

Set the floats carefully and slowly/gradually. Make sure the lock screw is always loose enough before turning the adjustment nut. If you strip out the bowl's threads on an older carb, you'll beat hell finding a replacement. 1/12 turn = 1/32" of fuel height IIRC. You set the fuel level, with pump running (wrap a rag around the carb so gas doesn't leak onto the manifold!!!) so the fuel is just starting to dip out of the "peep-hole" in the side of the float. Removal of the peep/seep-hole screw/plug is necessary first. Clockwise rotation of the nut lowers fuel level IIRC and counter raises fuel level. Again, please check their website. Most people get discouraged when dealing with the Holley Modular Carbs, but they are the simplest and the best carbs on the market. Even if 30+ years old.

Sounds like vapor lock. In laymans terms the engine heat causes the fuel flow to stop. Once everything cools off the truck starts. Insulating around the fuel line seems to be the solution. Try aluminum foil. Or perhaps a reroute of the fuel line to get it away from direct engine heat.

Most likely you have a Holley carb. You may have a 460 engine. If you have the carb with a big vacuum pot in front of the carb, then you have a RPM governor. The RPM governor will work against the 4 barrel.

The main mixture for the idle jets can be adjusted like any old style carb. You may have plastic caps over the idle screws but take them off. Then use a vacuum gauge on any intake fitting and proceed to read the vacuum level as you turn the mixture screws. You want the mixture to reach a maximum RPM, then evenly turn the screw in until the RPM drops slighty and then balance for smoothest idle.

After this step adjust curb idle and see if this helps when the 4 barrel kicks in. Holley has different springs for the vacuum actuated secondaries and they are color coded. If the carb is original it should be right, but if the carb is of unknown origin you can check the proper vacuum spring, main jet, power valve and so on in a good Holley book. The mechanical secondaries are also listed with clearances to hand adjust.

By adjusting the mixture screws you have set the engine to idle just below peak vacuum pressure. This means you should have more vacuum to work the secondaries.

sounds real easy either a bad idle air control solenoid or more likley a blown throttle body gasket. to test fro a blown gasket, with engine running and cool spray carb cleaner or brake cleaner around base of tb were it bolts to intake. when it hits the bad area, should idle down and be normal for a second or 2. iac is possible,try this first since its so common and let me know

The carb may be able to be rebuilt but those old *** carbs where not very good to start with. The Weber application is a great one and will last a very long time you can also get a kit for a holley if you want to get more performance. The engine in your truck will last forever as you already know but it will start leaking oil on the front side of the engine. When this happens the tech will try to replace the pan gasket but the problem will be the front main seal. When this happens have the tech change the timing chain at same time and you will save a fortune.