I think you will be fine running a 3.67 203 diff with a pair of 203 axle assemblies and leave the subframe alone.

I have a 3.67 203 pumpkin in the garage at the moment, (as well as a couple of 203' C-classes lol) but I'm almost certain the 203 pumpkin should slide right in. I just don't have a 202 diff at my fingertips to compare it to a 203 diff.

Also, since you have the speed sensor on your diff, I'm not sure if you can possibly tap into an abs sensor and grab the signal from there. The R129 3.69 diff I have does have a speed signal as well. But both are equally a headache (modify subframe to fit R129 diff vs. grab speed signal from elsewhere and install a 203 diff.)