I am missing some info on this topic. If any alternator has low output, a voltage drop test from B+ to terminal on the alt. running would be first. Also test grounds for clean, unpainted and use "Star" washers to help connections. If this is n "LS" swap, the alternator can be the 4 wire AD244. It is one of the best alternators on the market. Go the alternatorman.com to identify which alternator you have. They sell a voltage regulator for 1 wire operation. A 460 ohms resistor should be installed in series to ignition B+ pink, or in parallel with the 194 dash bulb. This is done to keep the alternator working if the bulb burn out. In parallel, the lowest resistance is the reading...both bulb & resistor are current limiting devices allowing 0.25 amps thru them. The OEM 4 wire uses 6 different regulators...GM uses 26+ regulators depending on year, make & model. That why they don't sell them.
As for slipping belt, i have a front alternator bearing lock up on a 2.8L GM V6 and engine would not crank over until belt was removed. Alternatorman.com has up-graded diodes, plate, bolt and regulator with a set point of 14.2 volt after 3 seconds of start up. LS alternators receive duty cycle signal from BCM, Charge lamp signal and 7.2 volts for supplemental brake booster vacuum pump on signal. Complicated, you bet your sweet Betty Boop! Which one do you have?