The 400 came out of a 4wd so it should have the correct tail shaft.
Not quite... there were 2 versions of the 400/205 - pre '85 and '85+. These 2 versions use different adapters,
different trans tailshafts and different versions of the 205 (adapter bolt pattern and input shaft lengths). If you decide to buy a 205 (to save cost / make the swap easier) keep this in mind so you get the correct tcase or parts.
You probably have a 205 with a figure "8" bolt pattern on the face for the adapter. If so, the 400's tailshaft needs to stick out ~4.5" from the tail of the trans case to fit a figure "8" version 205 with adapter.
I have no realistic idea of cost anymore. Once upon a time, the correct 205 could be bought for $400-$600 and the adapter was $150-$250 if bought separate. Nowadays, FB etc have everyone believing that even bald tires are worth $100 ea so pricing is crazy. Don't forget the cost of machining your tcase input shaft hole (which requires complete disassembly).
Off Road Design is a great source for the parts and they also have a really good tech page about tcases.