Drum to disc conversion questions

Pretty sure that's just a metering block (pressure differential switch). 4 wheel drum cars didn't use prop valves. You can use manual disc brakes easily and it won't be a problem to stop at all. I did it on mine. A power brake car doesn't stop any faster than a manual brake car. It just reduces the effort required to stop the vehicle. Your lines are looking pretty crusty, you might consider replumbing the entire system. At that point, just use your adjustable proportioning valve and you should be just fine.