Smallest wheels you can fit on a 8.8 drum brake rear. Explorer

factory ran 15" and 16" so i'd guess 15 is gonna be the smallest you can probably get away with. but if you had some BBP 14 stock steelies i'd certianly give those a try for S&G's

given the width of the rear and proximity of the spring, i'll take a WAG and say 4.5 BS is probably pretty close to all you're going to be able to get away with.

pay close mind to the center register, the ford may be different and the mopar rim may not accomodate that. i know on the front you need to open it up for several styles of rims, so i'd check before commiting.