Don Terrill’s Speed-Talk

Just an observation.
You mentioned that it does the same thing with a different carb so let's say it's not the carb.
If it is not the spacer are you not back to looking at the manifold?
Someone mentioned dams in the ports. Sounds redneck but it is proven to be a viable fix/crutch for a fat hole.
That may be your only recourse if a manifold change / test is not in the cards.

Does this engine have full floating pins?

I want to clarify a couple of things.

Dad ran this replacement engine without a problem.

You then put this engine in your truck.

You have changed carb, ignition, and transmission, correct? Was the flex plate changed?

This should isolate your problem to one of these three items.

I have had countless defective new parts. Dist cap and rotor almost had me pulling an intake off looking for a flat lobe before I decided to back up to square one and start over then finding 'no spark' on one wire.

I have a little inductive tach for lawn mower engines I just wrap the pickup wire on any plug wire and I get an accurate RPM. Turn the idle speed set screw up to your desired test speed then test each plug wire, they better all read the same.

Take the belt off the engine and run it, this eliminates possible water pump/alt/ps balance issues, I chased a vibration for ages until I did this and found it was the alternator was bad