KH disk front 83/4 rear, prop valve and master cylinder brake line question

I basically did what Dana describes, except I ran both front and rear lines through the metering block as per factory installations, then added the proportioning valve to the line going to the rear axle. This is very easy to do without cutting and flaring if you just buy a short (15" or so) pre-made brake line and bend it into a loop, then install the proportioning valve on an angle bracket parallel to the metering block, offset by an inch or two. The rear line just moves over an inch to go into the proportioning valve, and the loop connects the PV to the metering block.

This was on a factory drum brake 1967 Barracuda convertible — I duplicated the setup on a factory disc brake Barracuda (K-H discs and 10 x 1.75 drums), with the exception of using the 72-up metering block and Summit proportioning valve. I used a 15/16 disc brake MC with no booster. Once I had the PV dialed in the brakes were excellent. But I had to dial the PV out to something like 80/20 front/rear to keep the rear brakes from locking up. For this reason I would recommend an adjustable proportioning valve.

Rear lockup was never an issue with the OEM disc brakes on my 67 fastback which included a (purportedly) adjustable proportioning valve from the factory (which I never attempted to adjust).

BTW the pedal pressure is probably more on the order of 200 lbs, not 500 lbs.... I know you were just using a number for comparison, but I doubt many owners could muscle up 500 lbs. even with both feet. I typically do 330lbs on the leg press at the gym, and even the 1-1/32 manual brakes aren't at that level.