For rear discs you might be better talking to the current owners of Mulfab. IIRC Sierra Cosworth rear discs and callipers are used.
Talking to some experienced Morgan racers the five link rear is not the panacea that some may think it is.
For road use the drums are more than adequate , discs on a race car perhaps but only if you have more than 250 bhp. The rears do very little braking compared to the front , so spend the money on the latter.
The factory 5L on the early race cars ARV6 did have some issues , but as far as I am aware none of the the latest batch of 3.7 litre Roadsters (where 5L is a standard factory fitment) have complained about the same topic , so I assume it has been sorted.
For road use on an earlier car the SSL 5L system works really well , and I can personally recommend it. The ride quality does improve certainly , but the biggest plus is the much quicker and precise turn-in. That is what you wil notice first.
