rims, backspacing?

I'm about to buy these rims, and i need to know what backspace i need, and they are for a 1992 2wd ranger.... I need these to tuck nicely and i have no clue about which ones to get...

mine are 20x8.5 with 5.5" of backspacing.

what about 15" rims? i forgot to post the link to the rims, oops... http://wheelvintiques.com/shopsite_sc/store/html/page14.html