Yes, a 6" would be too narrow. What would happen is you would get severe wandering due to a lack of sidewall support. You could get away with 7" rims if you can't find 8", though. You will find in the long run, radials will win hands down. Longer life, better ride and handling over the bias plys. Also, bias tires have really gone up on price the last few years, especially in sizes and load capacities that these trucks need.
BTW, If you wan't to see what the Michelins look like, clink on my signature below, it will take you to pix of my turck, which has them on it.
