early a master cylinder swap

I think even with the aluminum adapter plate ($30), a later 2-bolt aluminum MC comes out much lighter. It was probably also less expensive for me since I bought a spare new one on ebay for only $25, plus a $15 rebuild kit. Aluminum doesn't corrode as bad as cast iron, though in the east it still turns ugly. The plastic reservoirs are probably better since they better seal, you can see the level without removing the cap, and they come w/ a low-level switch.

The latter, you can wire to gnd in parallel w/ another "low-side" switch. Ex., my 96 Voyager has 3 "grounding switches" which can turn on the brake light - reservoir level, e-brake, pressure imbalance. I forgot if my A's e-brake switch grounds. If not, you could have it light your oil pressure light, turn on the dome lights, or rig your own dash light.