Off the top of my head, my 78 axles with 33x12.5s on 8" rims with 4" of backspacing is about 80 inches in width overall. The 9inch is definitely 31 spline but should have the non-tapered outer bearings. The flat rollers work fine, but the tapereds (used 79 and up) are better and can be retrofitted with no mods. If you plan on using the radius arm setup up front, you'll be fine with a 78, but if you plan on anything else, try to find a 77 and earlier Ford truck which has the welded-on radius arm not cast-in 78-79. A quick search should bring up tons of info on them.
Jeep on!
--Pete
