I don't know that any published specs are available for horsepower handling capability, or really TORQUE capacity, for either transmission. But the M410 would be the better bet, since it was originally behind the big-6 B30 of the 164E. That engine made, predictably, 50% more power than the B20 it was derived from.
Grassroots Motorsports (www.grmotorsports.com) has featured, a couple of times, the SCCA Improved-Touring B (IT-B) National class winning 142S a couple of times- the name escapes me but I have the articles at home. The stories gave good details of how he set up his car within the rather stringent SCCA rules, and I believe he uses an M410 behind his built B20 "7000-rpm tractor motor". He also uses 240 rear axles for the strength there- I would think that's an upgrade you'd want to make too.
I did see one of these turbo conversion cars at the East Coast Meet in Vermont in October- here's the pic. You can see the angle the drivetrain is at in this one.
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Rob Bareiss, New London CT ::: '87 244DL/M47- 225K, 88 744GLE- 209K, 91 244 183K. Also responsible for the care and feeding of: 88 745GLE, 229K, 88 244GL, 146K, 87 244DL, 235K, 88 245DL, 236K