Look at the size difference between the alternator pulley and the crankshaft. The alternator needs to turn several times engine speed, so unless you're running interstate speed all the time, powering the alternator off the driveshaft isn't an option. I have run small alternators mounted on the transmission tailshaft, turned by a big pulley welded to the slip yoke, but that was on a dirt track race car, just to keep the battery charged during the race. We started out every night with a fully-charged battery, and probably didn't run 20 miles all night.
Jerry
