Newbie with Back Spacing questions

[85longbox]

Hey guys Im new to the forum and tried searching but didnt really find what I was looking for. So what I wanted to do was put on a set of Cragar V-5 365 wheels with 275 60 15 Firestone Firehawks all around but when I couldnt find any backspacing over 3.75" I scrapped that idea since I plan to do a 5/5 drop in the future and I want the tires to tuck with no rubbing. They do offer an Aero style D cut in 4" backspacing though so that may be a possibility but I need some clarification. I read that stock 15x8 rally's have a backspacing of 4" is this true, and if so is this the correct backspacing I need on the Cragar's to tuck in the 275 60 15 tires? The section width of the tires is 11" with a tread width of 9" and a 28" diam. when mounted on 8" rim. Im going for the Craftsman Truck Series look. Thanks in advance.

[westie 666]

my tyres are 11.5" wide and my back space 4" the tyres do rub so 4.5" would be better