Grind the inside of your case now, then all you have to do is put it all back together when the gears show up.
Towards the upper middle of the low range gear box are two oil channels. One on each side. Grind the inside edge of them until you get pretty close to the edge of the channel.
Even if they don't fit right away, you'll already have a head start on the grinding and it wont take too long.
It actually doesn't take all that long anyway.
A large grinding wheel chucked up in a drill press works pretty well.