A drum brake car does not use a proportioning valve. The metal block on the frame is a distribution block and a place for the brake system failure light switch, it does nothing in regards to proportioning the fluid pressure between the front and the back. Also, the factory drum brake MC does not contain enough fluid to allow for full wear of the pads without going dry, this is the reason that the disk brake MC has a larger reservoir for the disks.
When I did the swap to the 73+ disks I used a factory non power MC and the factory proportioning valve from the doner car. I also swapped in an 8.25" which gave me 10x2.5" rear drums (was going from 9" drums on all four corners). I replaced all the steel lines and rubber lines when I did my swap so everything was new in the braking system when I was done.