First off, I'm not a tire expert, but . . .
I know there are some smaller tires avalible. The problem with anything bigger than a 165 is the width of the rim. I was going to put some bigger tires (195s if I remember correctly) on the back of my wife's 75 convertible a few years back. Not 'cause that's what I wanted, but that was the smallest 15" tire WalMart had in stock at the time, and it had to have a couple of tires quick. Anyway, they wouldn't mount them on a rim under 5" wide . . . stock rims are 4.5" If you don't mind buying new rims, there are lots of aftermarket rims avalible that are 5.5" wide. You can even get a "stock" rim that is 5.5." That would open up a whole new world of possibilities to you.