I have a 65 Malibu that has had the F/brakes swapped over to discs. Supposedly the pieces are from a 70SS. I have that metering valve and all the related hardware, but the Master Cyl doesn't look like the ones I have seen in a 70 nor is it the bendix unit. Can I swap a late 71/72 proportining valve for the metering valve and distribution block in there now? And just what does the metering valve do for the system anyway??

You can use the 1 piece proportioning valve from a 71-72 on the system, just dont use the small proportioning valve that hangs off the master cylinder on the 70 setup. Run lines right from the master to the one piece unit and then from that run the lines to the front wheels and connect the brake line running to the rear of the car. the proportoning valve balances the brake load so that the brakes slow the car evenly, without it the disks and drums react at different rates and you would have lock up problems.