Have u done a compression check out of curiosity? I say swap in an 833 and see how it is. For the low compression big blocks a lot of guys like the Summit cams (6400 and 6401). The gear spread on an NP435 has no place in a hot rod truck. Even with its low compression (8:1 ish) a 400 in good mechanical shape should burn the tires right off that truck. I had a '77 warlock way back with a .030 over 9:1 440, crane 282/.450" lift cam, headers, stock intake and a tiny Holley carb that would get rubber at a 30 mph roll (auto with 3.23's). The intake u have is probably the best one you can get for your application.
If you're willing to do a stroker SB, for around the same money you could put 4.25 arm in that 400 and have a 512. You're already set up for a BB, I'd stick with that.
An Edelbrock 800 Thunder series is a good carb (I know it's easy for us to spend your money lol!)