Look please help..cragars and bs

ok..i have also decided to consider buying new 15" cragars....
but idk what bs i would need...
i would want 15x7's in the front
15x8's in the back..

the rear axle is from an old b-body,so is 1/2" longer on each side, so take this into account..

what size backspacing should i get with these...what woul be my safe ranges

i'm able to get ahold of 15x7's with 4.125" bs...
and 15x8's with 4.25" backspacing..will these fit without rubbing or lip rolling?