They don't bolt up. I presume if you found a used bracket somewhere, off the later motor which uses the bigger compressor, you could mount it. It may be more complicated than that.
It would probably replace the loss in efficiency of going to 134. Right now R12 is still the cheapest part in the A/C (well at least the sixth or seventh on the list), I'll keep it in my own car till it costs more than the compressor.
Bosch Master, ASE Master, L1
33 years MB technician