Pretty sure Peter is running 275/35/18's on 18x9's with torsion bars up front and custom Eaton leafs with a 1/2" offset kit in the back. He had been running 255/40/18's until recently.
If your intended application is autox or road race you really don't need the mini-tub in the back unless your plan is to be pushing big horsepower. Like over 500 to even consider it. The weight bias on these cars is to the front, that's where your steering is, and of course under braking there's even more load on the front. So, the front tires are really the more important consideration.
The other thing is that with a 3" spring relocation and minitub you're moving the springs closer together, so you need higher spring rates to compensate, plus adjust the length of your shocks. Sway bar length can be an issue too depending on the design, I know 72bbswinger was running an E-body rear bar on his car with the 3" relocation and mini-tub.
The super wide rear tires look cool and all, but from a handling perspective you want to be balanced, and the front tire is seeing the most loads. So unless you're hooking big power the staggered set up isn't going to make you handle better. In fact, if you don't have the horses to need a big stagger it can actually hurt your handling. I'm running 275/35/18's up front and 295/35/18's out back on my Duster with just a 1/2" spring offset. And really I don't need the 295's out back. I'd be better off rolling and pushing the front fenders a bit more so I could run 285/35/18's all the way around. That's a 26" tall tire so it'd be more work than a 285/30/18, you can run the shorter tire closer to the frame and still clear.
