Backspcing question for you big wheel guys

I am trying to figure out the proper backspacing for wheels. I am building a 70 duster, front disc and rear drums, stock wheel wells. I am going with 18x7 in front and either 19 or 20x8.5 in the rear. Does anybody know the proper bs for these cars / wheels and any suggestions on tire size would be helpful as well. I was figuring on a 225/40 in front and a 255/40 or 35 in the rear??


I have searched but there seems to be alot argument on the subject...lol