Need some tire size help please.

[JP2120]

I am rebuilding a 71 Super Cheyenne C-10 Big Block Truck and would like some advise on tire size.
I am changing my gear ratio and installing a posi unit so I need tire size first to determine my ratio. The truck has 15's with 5 lug and the tires on it now are 30x9.50 michelins.
I want to go with a sportier tire and also the widest possible on the rear. I am not opposed to modifying the rear suspension to accept a wider tire. I would rather keep the mods to a minimun though.
I have heard of people doing some kind of mod that was fairly simple but can't provide any detail on the mod. Something about doing frame mod and relocating an arm.
The wheels I have at the present are the factory Rally Wheels.
How are people putting 18" and 20" wheels on these trucks and not having rubbing issues ?
I am open to all advise.
Thanks,
Duke