Vacuum leak.....probably the throttle plate to the bottom of the venturi block screws loose, or the intake is leaking somewhere.
You are describing a large vacuum leak to a tee. Next time it fires, place a shop towel over the inlet of the carb. to choke it up a little, if it runs better, it's "stupid lean".
The biggest weak link on a BBC is the intake manifold to head, gasket surface. If they are not torqued properly (sequence, not tightness there Bam Bam)

, bank on a leaker...
I had a 454 burn three quarts of oil in 500 miles on my 3/4 ton 4x4, turns out a bad intake manifold from Weiand (Stealth), put on a new manifold and gaskets and problem magically disapeared, strange part is it ran really good...
GT