rims & tire size on 4.5 lowerd truck. (what size do you run

[1968GMC]

Im running 15x8 in front with 275/15/60 with (wagon wheels). Tires rub. if I get 15x7 with 255/60/15 tires will it take care of my probleme? The only other thing I was thinking of dooing was to replace my 2" drop springs with 1.5" drop springs. Keep in mind that I have 2.5" drop spindels with a total drop of 4.5" drop. What is the best back spacing for front rims? I think mine are 3.75 back space. I want to try and stay with 15' rims as they are cheaper. Please help.