I may be wrong, but your alternator looks like a 10si unit to me. They have internal voltage regulators. GM started using them in 1969 Corvettes and in all GMC cars by 1973. Maybe the mounting scheme and dimensions of your 10si are different than what was used on your '65 truck???
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
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