Download a copy of the parts and parts illustration books they're free and they're darn handy.
Looked the 73-78 NP205 transfer case in the parts illustration book. Looks like GM only used two possible drive gear part numbers (#65) in the NP205 for those years (plastic or steel).
The Driven (output) gear can be several. The illustration book page only lists a section number. You'll need to look it up in the parts book.
The NP203 illustrations are just preceding the NP205 illustrations in the parts books.
The parts books will tell you whether you can use the driven gear from the NP203 or not.
There are drive:driven gear tables in the front of the parts books that you can look up to find the part numbers for the proper drive and driven gear for your tire size and axle ratio. (assuming GM offered that combination for your year).
The below pages are as far as I'll dig right now. You should be able to find the stuff you need in the books.
