I have a 7.5 rear in my 5.0 auto and it's ok...however i plan to put the 8.8 in soon.

emissions would be a problem because it would not meet the standards required by your year car.

The rearend is different in 87-93 and 94-98 because of width, the 99+ is even wider- I wonder though if the driveshaft would still bolt up to your existing rear end? You would probably need a custom driveshaft...unless the 94-98 driveshaft is long enough, i know the fox driveshaft is not the same length as 99+ cars

whatever you do, keep the computer, engine harness of the same year donor car...and make sure whatever commputer you use is made for auto vs stick...stick shift computers cannot do automatics.