running problem

[mickman77]

hey everybody. what a great site. have a question. took the 400 sbc out of my '77 k20 and set a 454 in it's place. the 400 had a 3 line fuel pump and the BB has a 2 line. couple of guys told me to just cap the return line and don't worry about it. well, problem is, it will idle and drive down the road just fine but, when you slow down to make a turn or traffic, it wants to stall like it's vapor locking. feather the pedal and it stays running. otherwise, it stalls. no vacuum leaks that i can find, timed right, new plugs and all that. for the life of me i can't figure this out. would a vapor can hooked to the return line solve the problem?? please advise if you have an idea and thanks ahead of time for any responses.