All 300E's (3.0 ones) as far as I'm aware use the 722.3 transmission, all 190E's use the 722.4 transmission - what are the numbers on your autobox??
The boxes aren't *that* different, they're based on the same casing. I was interested in using my existing 190E box and installing a 3.0 engine - I've been told by an automatics specialist that the hydraulics are different, and even if you did get the engine + box hooked up together, it wouldn't drive well (something to do with slip angles or something
I'd be interested to hear if yours is a 722.4 gearbox.
Alternatively, maybe your car is a 2.6 6-cylinder? Worth checking the engine number - the original engine could have been replaced with the 2.6, which uses the 190Es gearbox, as does the M104 2.8, which was used around 1993.