The problem is that sources contain conflicting information in this case:
F15
= Alternator 65-amp [Scott Ross: "1970 Plymouth Road Runner", Forest Lake, MN: Cartech, 2018, p. 90]
= 75 Amp Alt [Paul A. Herd: "Charger, Road Runner and Super Bee Restoration Guide", St.Paul, MN: Motorbooks International, 1994, p. 316]
The 1974 Plymouth Taxi brochure mentions standard 65 amp (Chrysler) and optionally 85 amp (Leece-Neville).
A decision between the alternatives depends on the alternator that is advised for the Air Conditioning option, as described in the 1974 Plymouth Police Car brochure.