My 2 cents, Dave. I would ditch the 73 combo valve. Just use your 68 drum/drum unit it is just a switch to turn the light on, on the dash to tell you you have a problem, yeah thanks got that already. The 73 has same center, with a standoff valve for the front disc brakes, supposed to hold off pressure on the front brakes till pressure builds/equalizes in rear half, to help balance the system in very low traction conditions snow/ice/evil villain spraying oil on the road, none of which you should be worried about. I'm going to guess your Newport has 1 3/4"-2" wide brakes in the rear which should not need proportioned with the disc out front, if the combo valve even has one and if it still works after 45 years. If you do end up with rear brakes that seem too powerful and causing rear lock up problems (car will come around on you) you can always add a adjustable proportioning valve in the rear line later then a quick bleed and done. Then you will have adjustability to tweak your brakes for balance.
