Hi Geno, It seems that you have checked most possible causes. I am no expert, mostly fumble around trying to see what works and what doesn't. You should see a squirt of gas in the carb when you activate the throttle; if you see that, the accelerator pump is probably OK. You should be able to see the vacuum advance working by removing the distributor cap and sucking on the vacuum hose. The only other stuff that I can think of would be the throttle linkage hitting something and possibly shorting out the spark. And last --- one of the stupidest things that I ever saw; A friend pulled out to pass a vehicle and tromped on the gas, when he did, his truck died out! He went to his mechanic, and the mechanic asked if he had checked the oil level. Apparently there was a kill switch installed in the oil pan and when he tromped on it, the switch activated killing the spark.
Good advice Richard!There are so many possible things that can go wrong.But if you have good compression,ignition and fuel pressure and volume it should be all systems go.If the first is good then it's one of the others.That's why new vehicle's have freeze frame and monitoring capabilities.Makes it much faster to find the problem.
Geno, You said you checked the carb but did you change the carb gasket, had this problem with my suncrest with a 454 and a new carb gasket took care of it. Good luck.
With engine idling, starting fluid sprayed CAREFULLY around the carb should find gasket/vacuum leaks.