what size SBP wheels can we fit under a rear 1963 fender?

As info, I just mounted up my new set of 15" steelies. Fronts are 15x6, 3.75 BS, tire 215/65/15. They fit but it rubs a little bit at full lock to the right. Not a big deal, I can always back off a hair on a tight right-hand turn if needed. This could easily be just a function of a completely worn out front end that hasn't been rebuilt yet. The rears are 15x7, 4.25 BS, 245/60/15 tires. It fits, but it's all the tire I can get under there with stock suspension and no mods. Clearance in the rear is 1/2" to the spring, and I can get slide my hand fully between the outer tire edge and the fender. I'll get a better picture later if you need one, but here's what they look like.

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this helps a ton thanks!