I have a 1975 W100 Power Wagon, Extended Cab, Short Bed, 318/2bbl. I have already made some bolt on modifications to the engine and am wondering where I should start rebuildin next? I plan to use this truck for off road but also wish it to stay street legal.

Decide what size tire you'd like to buy as of now,buy them,put them on and take a Sawzall to the body for clearence. Wheel the shit out of it and if axles start breaking upgrade to ton axles. No use for big lift kits unless your wanting to look cool on a RTI ramp. Personally ran a 35"er on a 4x Ramcharger,stock,just cut the fenders and it went pretty good for being drove to the trail/wheeled and come home.
Just my $.02

Put a locker in the rear, it will give you more traction than just going bigger.