Keep in mind that I'm no historian or engineer. I just look at what they did, what the changed over time and theorize…. As best I can tell 65 model 273 had either setup. Those above, one a Dodge, the other Plymouth. I'm guessing the single belt design was their first attempt at a/c on LA. I think the single belt did work better on a slant 6 because its off idle rpm doesn't ramp as quickly. Without research I would say no engine they offered before the LA had so quick off idle rpm ramp.
If I had a choice I would want the 2 belt compumper drive. I don't believe a single V belt ever was enough to drive the V2 compumper on LA and their engineers figured that out. I imagine the vibration damper design in that a/c crank pulley had to do with tightening belt(s) tight as a banjo spring and/or absorbing belt bounce.
The heavier design alternator brackets of both early LA and the later model LA was probably relative to belt tension also. My 67 273 without a/c has a very simple tringle shape piece of 1/4 thick plate for alternator bracket. Earlier 273 had a heavier duty design of alt' bracket with or without a/c.
