Instead of swapping in a smaller alternator, replace your battery with a larger one (deep cycle) and put a kill switch on the alternator. That way the oversized alternator can charge the battery only when needed, like when engine braking. Check out these two threads:
Automatic alternator cut out/regen braking
Deceleration Alternator
If you want to address the fan and spinning losses that George Tyler mentioned, then try an A/C clutch pulley - it's been talked about, but I don't know if it's been done.
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e·co·mod·ding: the art of turning vehicles into what they should be
What matters is
where you're going, not
how fast.
"... we humans tend to screw up everything that's good enough as it is...or everything that we're attracted to, we love to go and defile it." - Chris Cornell
[Old] Piwoslaw's Peugeot 307sw modding thread