Rear tire size needed for 340 stroker?

I run 275/60R15 Cooper Cobras on 15x8 Cragers with 4.25" backspace and only have about 3/8" clearance from tire to leaf spring. But my rears don't rub one bit.
I see people saying 4.5 backspace fits all the time, but I don't think it would on my car...not without spacers.
Espo 1" over stock height springs, stock abody 8-3/4.

Now my fronts are 14x6 with 4.00 bs and 225/70R14 and they DO rub on the lower rear corner of the fenders if the wheels are turned on any kind of bump like a driveway. So for fronts I would go maybe 215/60...you might get away with 225/60. Not sure. I will find out what I'm switching to in Febuary.


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