There's more involved than just hoping it'll bolt up - which it does. The intermediate housing between the trans and Tcase, on a 203, is shorter than one for a 205. However, you can bolt the 205 onto the 203's intermediate housing, but be prepared for the following modifications:
- rear driveline needs to be lengthened
- front driveline needs to be shortened - don't ask how much for either one, because it'll be different for everyone
- If you have a C6 trans, and your front driveline uses a CV joint, then you'll need to trim the rear corner of the trans pan, to clear the joint. It can be done w/o any problems to the trans
I've done this, and haven't had a single problem with the swap, or any drivability problems since then