Wheels and tires questions

[mechanic350]

ive got some questions about backspacing and wheels that i cant seem to find a good answer to. i currently have boss 338 18x8 in front and i think 18x9.5 in the back on my 72 and im looking at swapping them out to american racing vn472's 18x8 in the front and 18x10 in the rear. im not sure on how much backspacing i need to get these rims to fit right underneath the truck. the wheels i have now have what looks like an offset of 2 and backspacing of 4.520 in front and then offset -4 and backspacing of 5.030 in the rear. any one on here that can help i would really appreciate it.