If the low voltage causes you concern, then change the alternator pulley. Alternator voltages aren't usually measured at idle. Power Master sends a sheet with their alternators that shows what voltage it was delivering and at what RPM. They also give you an intended RPM range to work in. Over speeding an alternator happens when mixing and matching parts, and no one remembers to verify alternator rpm. Sounds like you need to figure out what's the rpm range you motor spends most of it's time and then dial in the alternator speed to meet your needs.

As an example, consider that the pulley size on the alternator may be 1/2 the size of the pulley on the crank. So the potential exists to spin the alternator twice as fast as the motor is turning. When your spinning the motor at 5 grand, you could be spinning the alternator at 10 grand! They won't last long at that speed!

Street Rodder did a great write-up on this a few years back. In any event, figure out what rpm your alternator is running at (at idle) and if you have a local rebuilder available, check with them to see if a bigger or smaller pulley is on hand, make the change, then verify if your getting the intended results.