1972 20" Rim Back Spacing

[dhpenner]

Im sure the answer is out there somewhere.... but after a couple hours of searching, im throwing in the towel .

Im looking at buying some Boss 335 20" rims and want to know what the correct back spacing and width should be, front and back. I was thinking 20x8 up front and 20x10 in the back. My truck is a 72 SWB C10 with a 4/6 drop.

Thanks for your help in advance.

If you feel like leaving a smart a** reply or telling me to google it... please just private message me.