I received and installed the replacement alternator today and the problem is solved. Voltage while engine running is a bit higher on the dash gauge than before, and it remains consistent all the way to 6000rpm.
--'94 Caprice "SS" wagon, 0.30 over, high-volume oil pump, LT4 hot cam, 1.6:1 rockers, ported aluminum Corvette heads, shorty headers, PCM4Less tune, 4.10s, shift kit, side-exit dual exhaust with Flowmasters, OR X-pipe, F-body gauge cluster, Bonneville front and rear seats (working electrics), Bravada console shifter, Bilsteins, 12.7:1 Impala steering box, Boss 338s with 255/45R20s, bunch of other stuff I'm forgetting.
--'04 Mustang Cobra, '15 Audi S4
--1125R, 1050, VMax, VFR800, DRZ-400E