I had a 88 Turbo Coupe (Fox). I'm not sure of the differences (if any) between the TC and Mustang. But, I will tell you that the TC 8.8 was WAY wide for the Pinto. My recollection was about 3" too wide. Plus it lacks the spring perches you need to mount to the leaf spring. You can always weld them on but it is one more thing to do. Even if you get the 8.8 narrowed (if you even can) the cost will far exceed the price of an 8".

Regardless, here is some info that might help.  I got called away and didn't have time to read it all. Perhaps there are a few 8.8's (Explorer) that might fit???
 http://www.hotrod.com/articles/ford-rear-axle/


OK, had a little time to go back... . Read the link above because there are secondary factors rto consider! The '91 Mustang 8.8 rearend in 59.25 wide and later models even longer yet. The '69-'77 Maverick is 56.50", the Pinto/Mustange II is 57". The 90-92 Ranger are 56.50, and the 93-04 Explorers are 56.75 (note they are stated to have an offset that can be an issue with the driveshaft hitting the tunnel.  Here is the link to the source. http://www.carnut.com/specs/rear.html


I'd say the 8" is still the better option but if you go with a 8.8 know the aspects that are important. And, no I would not advise the '91 you are considering.

Similar to cutting wood when making alterations "Measure Twice, Buy Once." And, of course "Do At Your Own Risk." You have the spring perches to deal with, any offset in the trans tunnel etc. so, think it through. That 1.25" may not seem like much but can add up quickly. Even if you fit the wheels/tires withing the wheel opening it can still have that "front wheel drive" offset look.  Most Pinto's when they go to a wider tire offaset the wheel to the outside of the car.