The type of tire also has to do with keeping the tire on the rim as well as the width of the tire and the rim. If you are going to run an MT than the the 15x8" rim will be better than the 10". If you are going to run an AT or a softer sidewalled MT like the Xterrain, than you will want the 10" rim. I ran an 8" rim for years with 31's and 32's AT's MT's and never lost a bead, even when aired down to 2psi. Went to the 33" AT's on the 8's and just about blew a bead every other timne out, sometimes 2 beads on the same run. Since moving to a 10 rim, I haven't lost a bead in the last 6 yrs. A buddy of mine had the same problem running Xterrains on 8" rims, he would loose a bead on everyrun, now he runs bead locks. All in all a blown bead on the trail isn't a big deal, even if you loose both. You just jack up the truck and hit it with your onboard air and you are on your way.

As far as the 4.56's you should be O.K., providing your truck is still near the stock weight, you are only 2 tire sizes over stock. 4.88's will give you better mpg and trail performance. With my '90 3,0 on the highway at 80 mph the tach is at 3300, and at 70(where I tend to keep it) it is running at 2800.