I'd be very leery of using a disc on this frame as it wasn't designed to take that kind of stress. For one thing you'd have to get some really strong bolts for the dropouts.
I'd go the other way - use alu bolts and make them a deliberate weak point, like a bending fuse. Then the first thing to go is some easily-replaceable bolts rather than the frame.
If you have to fit a disc I'd go with the Hope C2 that has a force transfer arm attached to the brake boss, as this area of the frame was designed to take the forces generated during braking.
Except in a different direction and of a different magnitude
It's not unheard of for those arms to snap brake bosses off.
Even then I'd only go for a 140mm disc at the most and regularly check the frame for cracks at the dropouts and the brake boss.
Better safe than sorry
All fair points, certainly. Old frames + discs not always the route to happiness...