'88 F250 power problems

It's an '88 F250 4X4, standard 5 spd. transmission, the 351, a W block I believe, could it be an M block? It is weak. I Have to put it to the floor just to hold 55mph up hill on the highway in 5th gear empty, loaded it's really pathetic. My guess is it isn't getting enough air. After about an hour of highway driving it seems to clear up or something happens, because it starts pulling itself up hills with ease. It had new plugs, wires, cap and rotor a few months ago, helped a little, but just doesn't develop the power I think it should. Had an '82 ford van with 351W automatic, that thing would slam you back in the seat when it was full of stuff and pulling a trailer, this 250 just doesn't have it. Does anyone else have one of these trucks with this 5 spd. How does it perform? Any ideas on it's lack of power?

Thanks,
Rob

Its a 351W. They stopped using modifieds in like the early 80's sometime.

How many miles on it?