What size/type of stall or torque converter do we need?

My husband has a 1966 Plymouth Belvedere II. It has a 383 with an automatic 727 trans. We do not know what size cam it has but at an idle it wants to stall and it doesnt have the pickup that a big block should have. I just need to know what to get so he can install it. If you could post links I would appreciate it.