[Steeveedee]

You've pretty much eliminated the carburetor as the problem, given that the two tries made little difference. Changing the jets on the original carburetor is a mistake, IMO. It used to run OK with the originals, right!?

Pull the rotor out of the distributor and see what it looks like. If you have high resistance plug wires, the ignition can burn through the rotor to ground. It's an easy check, and is free. Back in the day when I worked on these vehicles, the rotor was the number one failure mode for an HEI. While you're in there, check the distributor cap for carbon tracking between the electrodes.