Here is the suspenion setup I have on my car:
H&R sport springs, KYB GR-2 shocks, ingals front camber kit, rear ($15) camber kit. I currenty have some 03 Eclipse GT 17" wheels on the car. They are 17x6, nice rims but the tires suck.

I just got some RX7 wheels 16x8 with Dunlop SP5000 225/50/16. These are the stock size tires from the RX7. The tires on the front rub a little when the wheels is turned all the way left or right. So i put them on and did a few launches they grip great.

But when I put the RX7 wheels on the rear they rub like Anna Nicole thighs. The tires are rubbing against the suspension arm that goes up and down. What size spacers do I need for the rear???

1/8th inch spacers did the job just fine for me, but if those tires are a bit wider you might need 1/4 inch spacers, which should still be fine if you use an aftermarket cone shaped lug nut which gives you enough turns on the wheel stud.