I need help on 1987 366 truck engine

I would suspect the carburetor, but, since the problem apparently remained the same when the carburetor was changed, it points to other things.

You may have a cracked intake manifold, bad intake manifold gasket, burned valve, or even a flat camshaft that is affecting one cylinder. Or, even a vacuum leak in an accessory that connects to one side of the manifold.

Each 1/2 of the carburetor feeds 1 "plane" of the intake manifold, which feeds 1/2 of the cylinders, every other one in firing order.

If there is a leak or a bad valve, it will be impossible to set the idle on one side of the carb, and all 4 cylinders fed by that 1/2 will be affected.

Check the compression of all 8 cylinders, and also, connect up a vacuum gauge, to see if the vacuum is steady, but low, indicating a leak, or pulsates, indicating a burned valve or other valve trouble.

Here's my fall back plan.
When I take a grain load to the elevator they like to dump the whole truck--it's faster--which also dump the carb into the intake and you have to treat it like it's flooded to get started.
So the rebuild doesn't work I'm trying to do a throttle body injection which (given I can get it running) will solve both problems and probably produce more horsepower allowing the timing advanced to be reduced.