I don't have an answer for you on the backspacing but if you go to the tire manufacturer's website, you can normally find the recommended rim width range for any particular tire. You can normally go a bit out of the range on the low side but going wider than recommended can sometimes be a problem with the bead not wanting to seal. Here are the specifications for BFG MT KM's:
http://www.bfgoodrichtires.com/specs...a-km/1152.html
If you go to Roger Brown's website (4Crawler) and scroll to the bottom of this link, you will see that the original "standard" rims were 6" with 3 3/8" backspacing. I know that my rims are 7" but I don't know the backspacing. I think 3 3/4 backspacing is o.k. but maybe someone else can confirm. Here's the website link:
http://www.4crawler.com/4x4/4R_TechInfo.shtml