Check that the brake compensator valve is not siezed - this will make the rear brakes do very little....
If you want to upgrade the rear brakes then this is the best option:
http://www.compbrake.co.uk/Fiat%2020V%20Coupe%20Rear.htmYou can also upgrade the standard rear pads to textars - which do help the rear lift on braking (these can be got from Alternative Autos)
If that is the purpose then you need some quality discs. I have used 3G 12 groove and Pagid 6 groove with no issues at all, and have given both of them a hammering. The DBA slotted discs are also very good, though I have not used them myself.
You will need a quality high temp pad like pagid blues or higher for that track, and dot 5.1 fluid if you are going to push the car.