I know I'm replying to an old post, but I bought a boat recently with a Westerbeke 27 and I'm facing the same quesitons you had last fall.
Did you upgrade to a 100 amp alternator, and if so how has it worked?
I have a 375 amp house bank, which would theoretically call for about a 95 amp alternator. I want to upgrade from the present automotive alternator and add external 3-stage regulation.
I know I'd need new pulleys to go to a larger alternator, and I don't know how the main bearings etc will handle the increased side load.
Another option is a 70-amp alternator, keeping the smaller belts and lower strain on the engine, and hopng that the overall charging time won't be too much longer.
What did you end up doing, and how has it worked?
Thanks for any info.