15" wheel size for 60 Valiant

You will be able to fill the front wheel arches easily. A 215-70-14 will be about the largest size you can go in in tire diameter (or 15"/16"/17" Wheel & tire equivalents) before needing to cut or trim the front lip of the fender to keep the tire from hitting. I cannot answer any backspacking questions for you as my 2 1/2 valiants all use stock SBP wheels. I used '67-'72 KH 4 Pistons discs and kept the 14" with smalls on my car.

Use this tool to help you out on your front tire size.

The rears will not care as much.
The sculpted quarter-panel lip follows the same shape as the tire (unlike the '63-76 Darts, for example, which have a flat top on the wheel opening that is not an arc). Just calculate backspace / offset so that you do not hit the spring with whatever wheel width you choose.