What Engine to go with

Go back and look at your original question.
You wanted an engine with a decent top end,for and occasional highway run.
Well a small block will do that. For the money, a small block makes the most sense.
By the way an A-body with a properly prepped 408 will wax an A-body with a 383.

An why would you stop at a 383 ? Why not step up to a 440 ?