The bottleneck is the air cleaner and filter. You can find a 5" tall air cleaner from a mid 80s smog truck and put it on there. It'll flow around 800CFM.
The stock system works very well. You can stick some retarded intake kit on there and it'll probably suck underhood heat. There are two good places for that truck to get air from. Its stock location or the passenger fender if you plug all the holes in the inner fender.