15" wheel size for 60 Valiant

Measure the backspace of the 13s you have now. Now measure the thickness of the adapter. Add the two together for the backspace you need, then add any extra width you want to the outside.
It I were to guess:..... 15x6 (preferred imo) or 15x7 with at least 4 1/2+ bs. (either one. )
And then.... a 25" tall tire, or less.
Don't spend a bunch on the wheels, cause after you do the disc swap, they probably won't work.
Best course of action, in my opinion? Hold off on wheels/tires till you've done the disc swap. Id call Dr Diff for the brake swap. I've always felt Wilwood charges way too much for what you get. Big name, more money.