I suspect an almost dead battery or other electrical problem, not the alternator. I put an electric fan in my 242, a big old Bosch unit out of a BMW and it doesn't affect the car much at all, even with the 55A alternator.
My car has a new battery, but before I replaced the old one I was expereincing serious voltage drop whenever I used a few accessories.
The voltage does drop just slightly with the fan on, but hardly at all, not enough to cause any concern. I have my fan relay powered directly from the output terminal of the alt, same with my upgraded headlights.
I have a small waterproof fuse board which holds 6 blade fuses down beside the radiator on the right. Each fuse is powered by it's own big wire directly from the alt, 4 fuses for the 4 relays for each headlight (quad square with E code light units), one for the fan relay, and one blank which will be for the foglight relay when I get around to rewiring them.
Angus