I know this thread is old and whatnot, but I think I found a permanent fix.

Stealth Conversions Mercedes V8 Parts

On this website they sell a machined flange that bolts to any 3 way yolk. (I'm sure it'll fit the trans yolks too). It allows a Dana spicer u joint flange to be bolted to it.

I've bought one for my 190e conversion since ill be using a ford style driveshaft with my v8 trans.

My thinking (for you guys) would be to buy two of these flanges, one on the diff and one on the trans, and have a custom made (steel or aluminum) driveshaft made to fit. I'm sure a custom ds wouldn't cost more than 4-500 buck or so.

Hope it helps