Chris
There are a number of issues here. You have to maintain a 60% front - 40% back, minimum braking ratio (or some times even more towards the front).
I fitted ventilated discs to the front of my '64 Bel Air and did not have to use a proportioning valve in the rear brake line by changing the rear wheel cylinders (the whole thing came as a kit and all the testing had already been done). It was a long time ago and I can't remember if they were smaller or larger diameter in the bore (someone out there will know). I also agree with Richard too and a proportioning valve might be the way to go. It will probably take some fit and test to come up with the right combination.
OLD 42
