engine lacking power what could it be?

[1972BackInBlackC10]

i have a 1980 gmc, it has a 350 4 barrel it starts and runs fine in park but when you put the truck into gear it loses power and when the trans shifts the truck dies

my friend thinks theres something wrong with the trans but iv never heard of an engine doing that over a trans, i was thinking its possibly the carb or a vac line unhooked somewhere

when you flip the throttle wide open quick in park it tries to choke out and die