How close is too close, Tires to leaf spring, backspacing

[dakman]

Couldn't find anything on a search anywhere so here it goes. Currently have a narrowed 10 bolt with 15x10x15 wheels with 3 1/2 backspace and 31x12.5 Hoosiers. There is 2 inches between the sidewall and spring pack.

I wanted to go with 15x12x15 wheels and 29x15x15 hoosiers. The wheels must have exactly 5" backspacing for the tires to fit between the fender and spring pack. Problem is this combo will put the tire sidewall 1/2 inch away from the spring pack that which leaves me with this question... how close is TOO close? Anyone have experience with this??