I did this conversion last year on a 1973 280 (W114) with an M110 and really pleased with the results. Installed a 135 amp GM alternator and IMHO it's made a huge difference!

I posted (somewhat) step by step instructions here:

http://www.peachparts.com/shopforum/vintage-mercedes/357901-replacing-55-amp-alternator-m110-engine.html

Purposely parked the thing outside in February of this year when it was -6 Degrees F, let it sit outside all day and then started it with the lights, fan and radio on as I cranked it over. Fired up on the first crank and the voltmeter sat at a steady 14.6 volts.

Worked so well I'm going to do the same conversion next week on my "other" European vehicle.