Hey those look awful familiar!! :evil Joe's truck really did scream with them! Good looking piping also! Hope they work as good for you as they did for Joe!
As far as the wheel size of the 80 I can't quite remember, its written down somewhere I will snag some pics of the hot pipe soon. The 80 is supported off the trans heat exchanger and I am going to build a mount off the alternator I think, who knows, I will figure something out this week....
If you run a cold air intake pipe down behind the bumper make sure you put an expanded metal screen or something in front of it. You're liable to accidentally suck up a stray cat walking along side the road.
That ought to make some air. Probably enough extra that you could add an extra wastegate on an electric solenoid to heat the cab and defrost the windows. Nice looking work!