I bought a 10SI and it seemed like the alternator body was too wide for me to be able to tighten the belt, and unfortunately most alternators are the same 5.25" width. I discovered the CS-121/CS-131 which were SI series replacements from GM. (SI is for "systems integrated" due to the fact that these were some of the first alternators to have an internal regulator, and CS is for "charging systems"). The 121 and 131 were the width of the stator (alternator body) in mm, making the CS-121 is about .5" thinner than the SI series alternators, but I wasn't sure of how well it would mount, and the alternators are harder to find since the few vehicles that used a CS-121 can take a CS-131 as well, so the auto shops only stock the 131. I tracked one down at an industrial alternator shop for around $200.

In the end, it didn't matter, because just as I was about to buy I found a company that sells an alternator conversion kit. They use a 10SI which supposedly has some simple case modifications (though I didn't notice any when I looked at it), and it fits and works fine, cranking out over 90A. Because of the high current, they recommend wiring direct to the battery which means your ammeter will only read discharge, but your charging light still works since it is on the field circuit. And presuming the regulator works fine, you can just look at the ammeter as a reading of how much power the car is using. Since the conversion requires a negative ground, you need to do the very simple ground conversion (see 2003/12/03 in the maintenance history), and if you don't flip the ammeter nodes, then this will just read positive anyways.

The shop I bought it from seems to have disappeared:

Power British
http://www.pond.com/~britcars/
Phone: 610-270-0505
I think the important part of the kit is a new belt, the proper pulley and spacers to push the alternator forward.

For how to wire it up, you can take a look at: Triumph Alternator Upgrade