Okay so I know the general consensus is to run an 8 or 10 inch wide wheel for a 12.5 inch wide tire. That said, I have a set of the old stlye SR5 wheels like these:
http://www.sonoransteel.com/1990_Mini/002.JPG
I'm planning on running 35x12.5 tires. Has anyone run this combo? As far as I can tell, the only negative is that the tires will wear in the center quicker. My only reason for running these rims is that I already have them at my disposal. Oh, and would they be too far inboard, ie too much backspacing?

 

run em! we run 35x12.5s on kmc enduro 15x7s on the race truck. doo it!

 

They will fit, although you may need to shop around for a shop willing to mount them, unless you plan to DIY. Center wear will be an issue, I run 33x10.50s on 15x6 wheels and have to sort of compromise on pressures to get somewhat even tread wear. I'll often run about 20 psi around town to get a flat contact patch and 40-50 psi on long highway trips (wears the center a little faster) and then 10-15 off road to wear out the sides of the tread.

As far as backspacing, that will depend on what sort of lift you have. Some lifts require less backspacing than stock. If you run into that problem, add a set of wheel spacers as needed.