I put a new slave on my Discovery 2 a few years back, when the old one just didn't feel right after fitting a clutch.
Externally mounted and good, old fashioned metal engineering with plenty of space around it, you could probably swap one at the roadside if you really needed to !
I did wonder then, if a 75 could be modified to something similar ? I think the fabrication of a pivot point inside the bellhousing could be the trickiest bit. Arm and cylinder could come from another car, and a bit of fabricated pipework....