If your small block is in good shape, why not just put a stock-type cam in it, like a brand new take-out Goodwrench 350 cam? They are commonly sold by crate engine vendors like Pace Performance for $50 including new GM lifters. And if they don't have one, I do.
Of course, it would help to know the compression ratio and head specs. A stock-type cam probably wouldn't like much above 9.0:1, and large volume intake runners (above 185cc) could be detrimental to low RPM torque.
So, for a few bucks and a day's labor, you would have something that should work in the truck while building a big block.
Ah, just noticed the Mexico heads. More than likely they would be good to go with a stock cam. Also, they're probably killing cylinder pressure with the big cam, which contributes to the engine's lack of torque. But, hey it's got that rumpety-rump idle.