has anyone ever welded a backing plate the the bolt flange on a rear axle, and can anyone think of any reason that it should not be done?

Welded a few on. all work fine. You can bolt them on if you wanted to make it a little easier to swap housings. Then again you could just torch it off and weld onto the new housing.

well the next swap will be a FF 14-bolt, i was able to fit 12-bolt backing plates onto a 14-bolt SF with some minor modifications to the backing plates, just didn't want to drill and have overlapping bolt holes, so looks like i'll be welding it