Jim,
I went with the Geo Metro alternator.  It is about
the size of a one pound coffee can, but shorter.  It weighs about 5# and is
rated around 55-60 amps and is internally regulated (I converted it to
external regulator).  They're also used on many other small
imports.  The only differences I've seen is in the mounting portion of
the housings.  The one I bought has tabs which match perfectly to the Mazda
water pump housing.

One more thing.  The pulley was only 2" diameter, so
I used a Honda Prelude power steering pump pulley which is 4" diameter.  My
crank pulley is 5-1/2" dia.  This will reduce the max alternator rpm's from
> 20,000 to about 10,500.  As you may have guessed, I'm running the
2.85 psru.  To use it, I had to remove the teeth from inside the hub.  These
are very cheap on ebay.

Let me know if you want to convert it to external
regulation and I'll send you a step-by-step on converting these.