I've got 275/40/17 on mine, fit like a glove, I rolled the fenders just in case but it never even rubbed before anyway
The widest I'd consider going with the stock 16x7 pony wheels is a 245/50/16...Anything wider is going to look like a balloon.....but with a 9" wide wheel, you could get away with a 275mm tire with little issue.
yup, just take a look at the 255 bfg drag radials, they bulge seriously on a pony rim. 7 inch are pretty narrow by todays standards.