try this. (nobody in front of the car please)
left foot the brake, give it a little gas get the rpms up a bit and put it in drive.
if it still kills the engine you have a converter locking up (bad sprag?) or possibly a trans problem.
if you can keep it running that way, and not kill the engine, then your problem is 90% likely to be the carb. possibly timing/electrical but i'll bet cash it's the carb. even though you tested with another one, they both could be in need of rebuild/adjustment.
the other possibility is a valvetrain problem with an intake valve staying open, or a crack in your intake somewhere. your engine will not keep vacuum too well and will stall out in either case.
