That is true, 16.5s are getting very hard to get. Most tire shop don't even stock them anymore. We don't at the shop I work at. If the truck isn't a dual wheel model, 16" rims should fit just fine. Used to run around with a guy that had a '49 1 ton, that's what he put on it. Used 7.00-16 bias ply tires on it. If you're not 100% worried about an authentic look, I would suggest either a 235/85/16 or a 245/75/16, which are very easy to get.

Bill Burmeister