BTW I thought the tubular UCA will take you out of ESP class. At least that was the old rules. If so, wouldn't bother and spend your money on rims and the most contact patch/grip you can get.
Oh I agree, only the same size front and rear. And that size would be 275/40/17's with like 5.72 backspacing. That's what the red 68 Valiant pictured below is running. There are some 17" mustang torque thrust aftermarket rims for under $150/ea I think. But the rear you need a spacer. OR you run a B-body rear end.
Is your car still raised some in the rear?? That doesn't help handling. How low is the bottom of your K-memember to the ground right now??