Yes, switch the carburetors and see what happens. Also is your vacuum steady, wandering through 3 or 4 pound sweeps or jittery? A compression test is a very good idea. Good to put your mind at ease. Check also both accelerator pumps. Good on the ported vacuum. Personally I'd hook vacuum back up to the distributor. I know there are folks who'll say you don't need a vacuum advance with HEI, but I'm not in that group. Very lean condition in addition to burning slowly can burn unevenly and that will also cause backfiring as rpms increase (won't all burn before the next intake stroke occurs). Good luck!

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end