i have 2 th350's. one of them has the 9in tailshaft but it is burnt out. the other has the 6in tailshaft but has all the gears. how can i swap the tailshafts from one to another?

Take the transmissions COMPLETELY apart, because the output shaft (part of a larger thing, actually; the low-reverse clutch drum IIRC) is the last thing you get to in the rear of the trans, as you tear it down; and put the one in the other. Piece of cake, should only take a couple of hours, not counting cleaning stuff up. Of course, you may want to get an overhaul kit while you've got it torn down, and freshen it up.

Then of course bolt on the extension housing that matches the shaft you used.