I've search for a long time and I can't find an answer this simple question. What is a safe backspacing for a 15x7" wheel on the front and rear of a 1968 Camaro with stock suspension and drum brakes?

My options are either 4" or 4 1/2" backspacing.

I'm thinking of running 235/60R15 in the rear and 215/60R15 in the front. What backspacing will work best for me.

Thanks

Either one will be just fine.

FWIW, I'm in the process of replacing the rear 15 x 7 (4" back space) GM Rally wheels with 15 x 8's on my 68 (to better accommodate 245/60 x 15 BFG Radial A/T tires). After purchasing 15 x 8 Rally wheels with 4 1/2" back space, I checked for clearance and found that the inside rim is about 1/4 inch from the lower end of the shock. I'm pretty sure it's an original single leaf rear suspension and stock axle. If you consider the addition of wheel balancing weights to the inside rim, it may be too close for comfort.
I'm trading in the 15 x 8 with 4 1/2" for the same with 4" back space. Hopefully, the 1/2-3/4" push of the tire to the outside won't make contact with the wheel well over bumps.
16 inch or larger diameter wheels would probably clear just fine with a 4 1/2" back space, but if you're considering 15 x 8", you may want to check that before mounting tires.

My 15x7's have a 4.25 bs and clear well with 235's but with 245's on a 15 or 16" rim, hits the headlamp bucket on turn bumps. Using the 16" custom wheel I find that I could have used more backspace as the inner wheel lip would be further away from the edge of the upper control arm. But these are on a 69.