I have a buddy who is going from 3.73 to 3.55 gear ratio since he went to a turbo setup and he's cool to give me his used 3.73 ring & pinion. These 3.73's have been in an 11 second Mustang. So should I go through the work of putting these gears in my car or should I just get new ones?
You'll have a hard time "breaking" a set of gears, but they do wear differently. Especially if not installed correctly they can wear the wrong way. This could make it really hard to install them correctly again and have them quiet, etc.
To me, it's not worth it. For the right price -- I dunno. Gears aren't that expensive to me, it's the installation and everything that goes with it that hurts. I've done it a number of times and don't care to do it more than I have to.