[dontlifttoshift]

Just an FYI. The 315s will work fine on a 12" wheel, no need to compromise the 335s.

General rule of thumb has been that you need 5.5" of backspace, minimum, to ensure caliper clearance to the frame rails. You will find that most minitub 1st gens are running 5.5 to 6.5" backspace.