Wheel Size?
 

I was looking at some wheels from this one company and I was wondering if 17x10.5 inch wheels would fit on the back of my stock 91 Lx 5.0.

No, not on your "stock" Mustang (read the recent post on "what a stock Mustang is", you'll enjoy it), but they can with modifications to the lip on the wheel well, exhaust, and such, it's not the diameter it's the width.

AFS Cobra Rs? I have em, and no, they will not fit nicely at all. I have a SN95, and I had to ditch my factory quad shocks, move the tailpipes, and it still rubs turning into parking lots and going over bumps in the road. I think it would look a llittle odd on a Fox as well.

What about 17x9 inch wheels?

17X9 is the optimum wheel size for Fox bodies. It's what I'll be going with once I do my 5-lug conversion. Be sure to modify the inner wheel wells in order to prevent rubbing. I beleive you can fit 275/17 R40's on them. I've been told anything OVER 275 is baaaaaad news. Not positive though...