It sounds like you don't have any rear brakes at all.
With that switch block and no proportion valve or combination valve you should have been locking the rear brakes up very easily. In rainy weather the truck should have been swapping ends.
When you replaced the front pads and lost the front brakes, that basically confirms you don't have rears.
I think you probably have air in both the front and rear lines. The rear shoes could also be way out of adjustment.
I would start with a new DISC brake master cylinder and build from there.
A disc brake master cylinder has one reservoir that is extra large. Usually the rear half of the MC is for the front disc brakes. On drum brakes the shoes themslves are adjusted to compensate for wear of the shoe lining. The amount of fluid in the reservoir stays nearly the same throughout the life of the shoes. As disc pads wear, the amount of fluid in the caliper behind the caliper piston increases. Without a larger reservoir for disc brakes, it can empty and you will push air into the lines.
I like the '73 up combination valve that you mentioned, better.
Some combination valves may require the metering valve pin to be pushed in for bleading and some require it to be pulled out.
When I put the disc brakes on my '67 I used all the brake lines, combination valve and MC from a '76. There is a connector for the rear brake line on the passenger frame rail just below the firewall or passenger floor pan. It's the same on disc/drum and drum brake trucks. I used all the lines from there forward. This mounts the combo valve on the cross-member below the radiator and out of sight.