Rim and tire help

[Jcfcamaro]

I have a 72 c10 shortbed I'm wanting to put 20's on it with 8in on the front and 10s on the back.I've dropped it 4in in the front with drop spindles and springs and 5in in the rear. Needing help with the correct backspacing for the front and rear. Also need some tire sizes that work the best. I've researched alot on hear and seeing alot of different combos. I see people using 255/40 and 245/40 for the front and 295/40 for the rear. I'd like the side wall to look the same for the front and back, but fitment is more important. I just want to get it right the first time with no problems. Any help would be greatly appreciated. Thanks Jared