The transmission is not your problem. Yes, have driven np435 truck. It's a good off road/work transmission, not really a good choice for a hot rod/race truck. But as stated, that cam is your problem. Who ever put that cam in there made about the worst cam choice he possibly could have. A stock cam would be a leaps and bounds improvement for you. That 284/484 cam is killing your cylinder pressure and your sub 3000rpm power. You should be able to lug that thing at 1000-1500rpm without problem. Also, I suspect your distributor timing is coming in too late. For a smogger BB, you'll want all timing in by the 2000-2500 range.