You have not answered the most important question in this problem.
'''Is the spark plug firing order correct ?'''
Go find Google 'CHEVROLET V8 FIRING ORDER'.
Before reading anything, click the IMAGES tab.
While standing in front of the radiator. Locate spark plug #1.
( #1 better be on your right side... or this is a FORD! )
Follow the #1 wire back to the distributor.
Is it plugged into the #1 distributor hole ? <- the most common misteak !
Flag it #1 with a piece of sticky tape.
Now standing on a stool above the distributor, follow the Google picture of each wire from the distributor to the '''proper''' spark plug. Flag each one as you go.
Flags in correct clockwise-order = 1-8-4-3-6-5-7-2
Now, it should idle like a Cadillac.