If converting all four wheels to disc brakes, is it still necessary to use a proportioning valve?? Thanks

You don't need a prop valve if you pick the correct parts. Classic muscle cars need roughly 2x the braking force on the front wheels over what the rear wheels have. You can achieve the 2 to 1 ratio by putting bigger rotors and bigger calipers on the front wheels. This is the typical solution that the OEMs use. Almost every OEM vehicle has larger rotors and bigger calipers on the front than on the back.

So if you are smart about it you'll copy the brake system from an OEM vehicle that is roughly the same weight as yours and has roughly the same weight distribution. If you do that you'll be golden. Unfortunately a lot of people just grab random parts from random cars and then their brakes don't work so well. I see it all the time and there is not an easy fix.

Feets did a pretty good write-up about brake math. Pretty sure it's a sticky. Start there.

But, I would plan on an adjustable valve for fine tuning.

If you are referring to what the OEM's used in a disc/drum set up the answer is no. Those do more than just proportion and it's those added functions that you do NOT want in an all disc setup. But if you are talking about an aftermarket adjustable type proportioning valve that only proportions then the answer is maybe. If you follow AndyF's advice, probably not, but if it is needed it would be to fine tune, not as a crutch for a cobbled together and misapplied setup.

Thanks for the replies. This is going to be on a 62 Dart/Polara. It will only be a touring car, no racing what so ever.. Even thou it is getting a 440 engine.

For a street car I highly recommend putting late model A body disc brakes up front and factory drums in the rear. That combination works great, is easy to maintain and it works. The 4 wheel disc brake stuff costs a lot more and most of the time it doesn't work very well. You don't need disc brakes on the rear of a street car.