440 what am I missing here?

Manifolds will work just fine if you want low end torque. A set of custom headers will add some power but not a lot from 0 to 4500 rpm. If you want to build headers then that is fine but keep them small. 1 3/4 at the largest. I have 1 3/4 inch headers on my Pro Street Duster and it makes plenty of power. Way more than I need on the street.

A small cam will help your combo a lot. I used the Mopar .528 in my Coronet with ported Edelbrock heads and HP manifolds. That engine would spin the tires all the way thru first and second gear. It made 475 hp at the rear tires on the chassis dyno. That was with a Performer RPM intake and a 800 cfm Edelbrock carb. A cam in that range would wake up your truck.

This is all assuming that you don't have some other big problem with the engine. I still think you need to double check everything on the engine before you start swapping parts. If the engine was put together by someone without a lot of experience then there are all sorts of things that could happen. The cam can be retarded a tooth, the firing order can be wrong, the rings put in upside down, etc. The list of possible mistakes is endless......