R and Z Alternator in Harbor City, CA makes an aftermarket alternator for a 1500, you can get them from 40 amp to 85 amp, the alternators range in cost from about 130.00 for the 40 amp and 180.00 for the 85 amp. I installed one of these on my 98 and have had no problems with the alternator at all. At idle it sits at 13.7 volts and at 3000 rpm at about 14.6 volts. They are the same size as the OEM alternator so install is a breeze. I talked with Robert (the owner) and was curious as to how they can get twice the amperage from an alternator the same size as the OEM 40 amp. He told me that the stators are double wound and the rectifiers/regulators are rated at 80 amp. The configuration of the windings are in the "star" congifuration and are 3 phase, this produces smoother voltage overall and higher amps at lower rpm. I'll keep you posted on the performance of this alternator, I've been an electronic engineer for 40 years and I know that in electronics you get what you pay for, there are a number of ways to cut cost in electronics and allot of time the consumer pays for it. With that said, I'm trying out his alternator and hoping for the best, so far so good, it hasn't even falted once. I know a few guys in our GWTA chapter that have went with the "other more expensive one" you know which one, they say they will never use it again, one of them failed at 500 miles and the other failed after 1500 miles.
One thing to remember when installing a higher amperage alternator, one "should" upgrade the white wire from the alternator to the battery, remember these were designed for a 40 amp alternator, although, you may not need all 85 amps you should have no real problems with the white wire heating up under normal load, but if your load increases to over 50 amps the white wire my start to add some resistance and heat up.