1.
What is the fuel pressure?
2.
Will the ignition system reliably fire a spark-tester calibrated for HEI? There are several kinds, some of which are practically useless. This is the kind I prefer:
https://www.amazon.com/Performance-T...=3HE3HUCPW3QFV
If it won't fire the spark-tester at the spark plug end of the plug wires, flip the distributor cap upside-down WITH THE WIRES STILL CONNECTED, and try from the center button. Use the spark tester, a grounded jumper wire, and a cut-down spark plug boot.
http://hbassociates.us/HEIsparktesterandboot.JPG
http://hbassociates.us/HEIsparktesterincap.JPG
The carbon button in the center of the HEI distributor cap should be replaced with a low-resistance aftermarket button/brush.
https://www.summitracing.com/parts/fst-730-8412
MSD 8412 is the same thing.
Accel has--or had--a similar product.
There are a dozen other possible causes--plugged exhaust, incorrect ignition timing, massive vacuum leak(s), dead cylinders, failed torque converter (on an automatic-equipped car), dragging brakes, etc.