B2600 - Wheel backspace for 31x9.50?

I want to put some Super Swamper TSL Radial 31x9.50 on my '91 B2600i 4x4. They recommend a 7.5" wide wheel. Is stock a 6" wide? Is stock bolt pattern 6x5.5"?

What backspace do you think I need to run if I switch to some 15x8" steelies?

Thanks

on my 1990 b2600i stock alloys are (i believe) 8.46 (8.5) x 15 and i had 31 x 10.5 b.f. goodrich all-terrain tires without any problems.


also, here's a pretty good tire calculator that also give you tires in the sizes you're comparing.

http://www.1010tires.com/TireSizeCalculator.asp