I just went through this whole 9-1/4 narrowing exercise couple months ago. There was a Mopar Action tech article a few years back and it looked like a good option to a Dana. However, NONE of the big shops out there want to touch one. If you have the time and inclination you can probably do it yourself - all the good parts as well as axles are available. Check youtube - there are a couple good videos about narrowing them.

If the thing needs a full rebuild then I'd just opt for an 8-3/4 if it will handle your power - and anyone will narrow one of those AND you don't have the c-clip issue (which really isn't an issue in the Chrysler axle).

Will the backing plates interchange? YES

In the MA article they swapped out an 8-3/4 for the 9-1/4 and had to shorten the driveshaft. So if you swap in an 8-3/4 you'll need a longer one.