I can’t tell you what works but I an tell you what does not. Dales Restoration has sold rear disk kit that bolted on with no modifications.
The problem is that they do not provide enough braking power to provide balanced braking on the car. I’m guessing it is close to 90% front and 10% rear when it should be something like 60/40. What ever the ratio is, it is a serious problem in that standing on the brake pedal as hard as one can fails to lock rear wheels up, even on gravel.
I have tried a variety of pads with some noted improvement. I eventually put a proportioning valve on the front calipers, of all places, to reduce front braking. This provided a more balanced system. Of course the pedal effort has increased. The problem with Dale’s set-up is that the caliper piston is too small. Fiat makes the same size calipre with a larger piston that will about double the effective force on the pad. The only problem is that there is no emergency brake. I’ve tried them and they work great.