Is it the extension housing bushing number 96?
Good idea. If that's what it is, you should be able to insert it in the tailshaft bushing then slide it over the tail shaft when bolting up the extension housing.Could be, try it out like that to be sure. If that's what it is, it would be handy to have when pulling a trans out.
I think a trans swap would be no less involved than replacing the tail shaft extension housing.Excellent! Now you can keep it for trans swaps.