question on a 90 amp vs 120 amp alternator

Bigger crank pulley is the easiest, like Kim said. But if you're stuck like me (fluidampr, only one pulley that fits and it's an under drive) then there are options.

If you have the mid size denso, 90A from a dodge truck, it uses a 17mm shaft. I ordered this pulley, but it required modification, bored out to 17mm, machining to clear the housing and a shim behind the pulley. If you have access to a lathe it's a fairly simple fix.

Unfortunately, in the 17mm shaft size, there are limited options for smaller diameter pulleys

This gave me 14.1v at 850rpm

http://www.metroautoinc.com/new/index.php?route=product/product&product_id=1007