That is a pretty close match...I don't have any experience with blowing beads firsthand, but I would say with a setup that's that close in width it is definitely more likely to roll off the rim than if you kept the 31x10.50's on your 15x8's. Also, in my opinion, it looks way better if the tire isn't flat on the side, a little bit of balloon effect is nice. The balloon look also offers a slight margin of protection to the rim and hub that way.
My personal experience that is somewhat similar, was mounting up my 235/80/17 (32x9.5/17) BFG KO's to a set of 17x8" rims. It was a huge pain in the neck getting the beads to seat because when the tires are brand new the beads are like 3 inches apart! I don't know if they would still be like that after being in service for a while but I can only imagine trying to seat the bead in the bush in inclement weather...would not be enjoyable. My advice whether you run the 8" rims or the 10" ones if you are airing down close to 10 psi...bring a compressor, a can of brake clean or ether (and your brain), something that'll throw sparks, a jack (duh), and a sturdy ratchet strap you can cinch around the circumference of the tire to help it seat up better.