How would this Alt. perform with underdrive pulleys?

Poorly. Very poorly.

That was a 70 amp alternator on the best day it ever had. Reworking the stator for higher amperage trades low end output power & peak voltage for more amps.

This means it does not put out much at low speeds, then you cripple an already limping alternator by driving it even slower. Even spun up, many of those "High amp" alternators have trouble making more that 13.5 volts.

That old mopar alternator was a trendsetter 45 years ago. It was a 37 amp unit. The very last of them were 70 amp units. So called "100 amp" versions of that unit are highly suspect. The slots in the stator only have so much space. To carry more current, they wind it with heavier gauge wire. So there is no way to have as many turns through each slot with thicker wire.

What sort of current loads do you have that make you want a 100 amp alternator?

B.