Looking at installing a Mercedes fan in my '68 Newport. I have a 90 amp Powermaster Alternator, and 2, 10-gauge charge keads running from the alternator to the starter relay. The ammeter bypass has long since been done, even my headlights have their own harness and relays. My question is then, with only 100 amp high beam headlights and the Mercedes fan as my only high amp draw accesories, will a 90 amp alternator keep up?

it should be fine.
I ran it for a while with the m-body 78 amp alternator and didn't have any problems.
the only problem I had was I put an inline fuse that was one of the glass ones and it melted the holder. seems to have been a known issue with high constant draw on those holders I read later.