Rearend... Tires... HELP

you could probably fit a 245 tire. but please take pics of you towing a 32ft camper

Pony rims on a saleen? There are sets of American Racing 5 stars that are meant to go on that car on ebay fairly often.

Then you are considering towing a camper with it?

Well, you asked if it is absurd, and my answer would be yes.

The Saleen Mustangs typically had something other than the Turbo Coupe rear axle. The axles were longer and the disk brakes were different. The Turbo Coupe axles were 3/4" longer per side and the disk brakes were a 10 5/16" vented rotor. I am not sure what the Saleen had, but it was different.

You will want to do a search on 91 Saleen and see what you can find.

Towing with mustang= bad idea... Especially a 32ft trailer

The Fox Saleens used the 5 lug SVO rear disk upgrade. Standard rear wheel and tire sizes were 16x7" on the wheels and 225-50-16 for the tires.

The SSC's used 16x8" DP wheels with 225/50-16's up front and 245/50's in the rear.

However if you plan on towing (NOT RECOMMENDED) a 32' trailer with your Mustang , you had better leave the smaller 225 on the rear.

The rear of that car is gonna sit down on the tires.

You will completely kill that car attempting to pull a large trailer like that. You'd need upgraded engine and trans coolers. Beefier clutch and tranny, and a Bigger rear axel with at least 3.73's. You will smoke your brakes.
Also, unless you install an electric brake controller, you'll never stop a trailer with your Saleen.

Your question Absurd or possible?:

Possible but Absurd!!