What dictates alternator amperage?

[Detour]

My 67' K20 327 has the factory style alternator that is stamped for 37A. I found a guy locally selling a NOS 55A for $10. I grabbed it and didn't look in the old greasy box until I got home. It is definitely a heavily used alternator that was remanned in the past. (Heavy aluminum color overspray in all the vents). I pulled it apart and it was full of crud and dust from the brushes.

Thing is everything looks identical between the old 37A and the recently purchased 55A. Housing, diode/electrical components in base, stator and rotor even appear to have the same # of windings. What components should I swap into my OE alternator housing to up the amps? Thanks

[Dead Parrot]

At this point, I wouldn't trust the 37/55A stamping as being accurate. I doubt the rebuilding places paid attention when reassembling units. When I was a bit younger and playing the alt parts swap game to save money on a 68 Impala, there was a noticeable difference between the 37 and 55A stators. Don't remember if there was a noticeable difference in the rotors.

Unless you are wanting to keep your truck original, I would swap to a newer style 12SI with the internal regulator. Should be a simple physical swap with some rewire needed. Plenty of threads on this forum on doing that. You could drop a 70ish Amp unit in and it could loaf along powering your truck. Bonus is it will look very close style wise to your older unit.