The best thing to do is talk to George at Transfer Cases Unlimited.
But from what I know putting a 231 in place of the 249 the things you will have to change and watch out for are:
1. Rear drive shaft will need to be modified (esp. with a SYE)
2. the 4wd indicator switch in the transfer case is it 3 pin or 4 pin?
3. The shift bezel will need to be changed too.
it depends on the flange on the front of the 249 and what comes on the 231 as this might have to be changed too. But like I said upfront talk to George at TCU and he can get you a good deal on a 231 with a SYE installed. Here is his email transfercasegeo@AOL.com