I'm toying with the idea of building one of my 86 F250 super cabs into a service truck for my HVAC business to replace my 04 Ram. It's 4x4, I'll probably use an E4OD and it currently has 4.10 gears and 33's. The tires are shot but will stay roughly the same size.

I'm on the fence about the induction. EFI is a natural choice, but I'm not a fan of how ford programmed the E4OD so even though it's more money I may go with a stand alone controller and a good old 4bbl carb setup.

What rpm should I target on the highway at 65mph? The truck will be kind of heavy as in thinking of a utility bed and I usually carry about 1,000-1,200 worth of tools / equipment with me. A v8 I would think around 2k rpms but I'm thinking 1,600 or so with the 300.

Depending on finances I would do some work on the 300 like porting the head, Chevy 250 rockers, etc. Maybe even a small cam.

 

My stock F150 with 3.08 rear gears delivers good economy while driving 62 mph @ ~1770 RPMs. The engine is quite relaxed and handles hills well.

However, that is a relatively light weight short wheel base and 2 wheel drive. It tips the scales @ ~3750#. So, I would think you might want a little higher engine speed.