They actually do have 17's available now.
On the large dia. wheels like the 16" & 17" magnum 500s, it appears the spoke/bolt center section is the same used throughout the size ranges, they just put a larger outer ring section on for the larger tire. This changes all the nice proportions of the Magnum 500 giving more chrome ring when viewing. (...)
Here's an interesting shot of some custom 17's on a '65 that were on a recent ebay car. I don't think they look too bad either. Note that they kept the wheel rim proportions the same as a 14/15 but increased the center bolt/spoke dia. size, which is opposite the way Mag 500 17's are made. If they made a Mag 500 with the same proportions of rim/spoke as 14/15s, they might not look too bad but to my knowledge, they don't. Anyway, these were interesting 17's and I thought I'd share them with you.
Used Cars for sale - Ford Mustang classifieds - Forum Camaro FR