Sounds like yer making a slower-on-purpose turning alternator do double the work.
I'll bet your alternator's voltage regulator glows in the dark.
That's where the failure prolly is.
Heat is killling it most likely-heat from the excessive load.
I was you I'd get a heavier alternator/regulator and put a bigger diameter pulley on that alternator so IT will spin at stock speeds even though you got an underdrive pulley on the crank.
Calculate the maximum current load you'll be pulling through it and go some over that when picking out an alternator.
You can't expect the alt to 'idle' and provide maximum output.