Jerry -- It also depends on your opinion of "looking good."
I'm running 17" multi-piece wheels with tubed bias plys on my stock '49 3800. The tall tires (7.50 x 17 to be exact) give good speed (about 50 mph if I push it) and look right on the truck (to me). IMO, 15 inchers would look silly on the big pickup and, as the others have said, would rob you of top-end speed.
Note: My truck is NOT a daily driver and if I take it out of the County, it goes on a trailer (Not that it's a pristine show queen, which it is most certainly NOT but I'd rather keep the miles off it and I don't have a lot of spare time to waste traveling).
