235/75/15 XL would be my first choice. Lots of options, relatively common size, and most tire shops have plenty on hand. Please note that I listed it as the "XL", which stands for Extra Load. That tire is a bit heavier duty than a regular P rated tire, but isn't quite as heavy as the LT C range. Nice comprimise. You'l still get a decent ride, but it'll still carry the weight of the truck and an occasional load if desired. If you want a bit larger, you could also consider going with a 30x9.50x15, which will be ok on the 7" rim. I have that size on the '55, and am very happy with the performance. I am running an 8" rim, however.
