Bergy, I ran into the same issue recently.
The 68 Camaro subframe from a big block car that is under my son's LOS 1967 327 car has the same thing. Passenger side mount is taller.
Heater box also changed to big block type. Car hasn't been messed with since 1989, but who knows what happened before that. I haven't had time to dig deeper. But it is puzzling.