How about dropping from at 100 amp plus alternator to a 75 amp alternator? By definition its maximum output would be less. I suspect 75 amps would be sufficient to drive the ignition and the cooling fans without difficulty. .
That's certainly an option, but it's hard to say if it's worth the trouble if you already have the larger alternator. I say that because I wonder if you'll ever really be drawing more than 75A from that 100A alternator. It would be worthwhile to add up all the possible loads to see, and that's the usual recommended approach to the problem.
But if you're really free to choose alternators, here's another more complicated way to look at it....
Alternator sizing is a subtle subject because in reality you don't really have to have one who's output equals the sum of all possible simultaneous loads. When the loads exceed the output, all that really happens is the battery discharges during the period of excess load. If the excess isn't terribly great, you survive that just fine as long as you don't go long enough to fully exhaust the battery.
To illustrate (Here goes another back-of-the-envelope analysis...) let's say for the sake of argument with absolutely everything on you draw 85A with a 75A alternator, and you have (say) a 50 amp-hour battery. That's a current shortfall of 10A. That means that under those conditions you can drive for
five hours straight before the battery dies and the car stops (or more accurately, refuses to start after stopping). In reality that's unlikely because (assuming typical GT40 electrical loads) you'd have to have the radiator fans
and headlights on the entire time to get into that situation, and that's simply not realistic unless you drive in circles around lower Manhattan on a hot summer day during a total solar eclipse.
So I guess the upshot is yes you
can lessen or eliminate the probability of belt slippage by going with a smaller alternator. Having done that you could mitigate the impact of the small alternator by putting in a bigger battery if you want. In reality I suspect we would all be fine with alternators just big enough to drive our headlights and our ignition boxes (that would be something like 20-25 A) as long as we keep the car moving and thus the radiator fans off.
So probably the thing to do is make a careful list of all the electrical loads and try to imagine what is the largest collection of those that will be operating for more than (say) an hour at a time, and then use that as your alternator size.
By the way, given my somewhat ridiculous space constraints with my FE big block, I chose this incredibly cute little alternator made by Nippon Denso (93 mm = 3-3/4" in dia!):
http://www.qualitypowerauto.com/catalog.php?item=165
