Use 4.5 inch backspacing if you go with 15 x 8 Welds. I just put 15 x 8 Centerline Telestars on my car with 255/60/15 in back and 235/60/15 in front and had no problems.
I had 16 x 8 Wheel Vintiques ralleys before with 4.5 inch backspacing and 245/50/16 tires all the way around. No problems with those either.
1969 Camaro - Intense Blue Pearlcoat/white (Baldwin-Motion paint scheme), stock 6.2 liter L92 with VVT, LS3 intake, Doug's Headers; Tremec TKO, 3.73 12-bolt posi.