The standard 2wd 16v were available with an ABS option in Australia, although I'm yet to come across one with ABS fitted. This would be the easiest option to fit rear brake discs.
Someone on ebay was selling some 33 brake conversion parts in the UK and I got in touch with him - here are some photos of some mods that he did for a 33 axle. Basically you just need to design and weld on a suitable bracket to hold the calipers, and fit the appropriate hub and disc. The choice of caliper is important, one with a handbrake mechanism is required, unless you can get away with a hydraulic handbrake. Alfasud front calipers might be a good choice, but one would need to create modified handbrake cable system to engage them. Remeber on the Alfasud they are mounted inboard close to each other and the handbrake mechanism simply pulls the two leavers "in". If you've had a 'sud engine/gearbox on the garage floor, you'll know what I mean

Alfetta rear calipers might be another good choice. What about rear Alfa 156 calipers? Are they twin piston with a handbrake mechanism?
~Benjamin