I've traveled 100's of thousands of miles with the NP435 in a working PU. 1974 D200 with a 360. That transmission out lasted two engines... never opened up. Yes it is functionally a 3 speed in normal driving... 1'st is for creeping or getting started with a heavy trailer. That truck was peppy even fast for its day with 4.10's and ~29" tires. The NP 435needs matched up to an engine with wide torque curve.

Agree with all the comments... low compression, giant cam and tall gears do not work together.

The only thing to add, is have you considered building the 400? Make it a 451 with a 440 stroke crank and you could have a superb engine. You already have almost everything else (headers, accessories, manifold ect.) and BB Ramchargers are pretty special. With a 451 built for torque, the gear spread on the transmission and 3.23 gears would not be an issue at all. Different truck.