Coys Backspacing Question

[dhart]

I have a 72 c/10 that is lowered, drop spindles and 2 inch lower springs in the front, 5 inch springs rear, adj trac bar, shock relocators, sway bars etc. The truck has all stock width as far as control arms and rear diff and it has the inner fenders.
I currently have a set of 17 inch TTII's 17x8 & 17X9.5 with 255/50/17 & 275/50/17 inch Nittos.
I am wanting to put a set of Coys C-5 20 inch wheels on it, and i am needing some help and or pics of backspace reccomendations for a 20X10 inch rear and 20X8 front.
It seems that the rear spec seems to be 20X10 with a 5' inch BS but i know a bunch of peaple disagree on the front.