Re: Fuel starvation?
Just "Blue Skying" some ideas here, don't replace anything without testing it 'cuz of me!
I assume that there's no computer on it, only an ignition module, carb is a big 4 barrel, and ignition system is in good shape.
1) Converter partially plugged as Norm Speed mentioned - Easy to test with vacuum gauge by running engine at high (~2500RPM) idle for a few minutes to see if intake manifold vacuum falls off vs. time due to pressurizing exhaust system.
2) Fuel Pump - What is pressure mounted on engine? (thinking if cam lobe for fuel pump was rounded off, press. would be low installed).
3) Crud in tank floating around and getting sucked up against "sock" filter on tank pick-up - I had terrible trouble with a Olds 455 once because of that. But you have new tank, and if the pick-up was crud-covered, would think you would have noticed.
4) Fuel Filter - Is it an in-line between pump and carb? The big one with hose fittings? Picking up heat from the block? Could try insulating the outside of hose with some insulation with reflective-foil on the outside. I have used pipe-wrap with a foam core, heavy foil on outside. Not fire-retardant, so keep off of the exhaust manifold!
5) Carb Venting - Was there a carb bowl vent that's not connected or plugged now? (If heat from engine vaporizes gas in the bowl, it needs to get out, or it can fight against the pressure of the fuel pump trying to pump liquid gas into the bowl).
6) Heat in general - With a big-block in there, is radiator larger to handle? Or is it really cooking? And any fuel line subject to high temperatures can become a gas generator. My big-block Olds had a 3 port fuel pump - 1 in, 2 outs. 1 out was a fuel bleed to a line back to the tank. This cycling of fuel kept the pump cool (relatively speaking). Worked great, would not vapor lock even idling for 30 minutes at 98 outside, stuck on expressway @ 0 MPH, with A/C on!
7) Gas tank and/or gas cap venting - No, you covered that by trying without the cap.
8) Here's an idea all the way at the other end of the spectrum! What about carburator icing? Is there a working air pre-heater, and a working flapper valve and temp controlled vacuum in the air cleaner? Tube connection from pre-heat exchanger on exh. manifold to neck of air cleaner in good shape?
These are a few things that came to mind quick. I'll add to them if I think of more in time.
Everybody else, join in if you've got some ideas!