I may be the last owner to need a heater valve rebuild but in the event I'm not thought I would share this with you.
I decided to remove and inspect the 67 PG 18459A-A heater valve in my car since the hoses were not connected when I purchased it and did find the shaft seal had minor deterioration. I've read this is normally the cause for leaks. There was no reference to the shaft seal when searching the forum or web. I eventually found a 1/8" x 3/8" x 1/8" replacement V seal part # 870-41 for $1.18 at Hercules Sealing Products. This seems to fit properly and I'm sure will work. The plunger outer flange seal was fine and can't imagine finding one of those anywhere. The end cap seal is a standard 1" x 7/8" x 1/16" square o-ring.
This beats the heck out of paying $157 for a rebuilt valve plus core and shipping.
Hope this may help someone.