There's still several companies that rebuild the hydrovac assemblies on these old trucks. A search through the "big bolt" subforum should yield all your answers.
There's also a few threads on here specifically about 1967 brake part numbers. I've posted all the part numbers before for brake hoses, wheel cylinders, shoes, etc. I'm on my phone otherwise I would try and find it.
https://www.stovebolt.com/ubbthreads...big-bolts.html