I want to finish off this thread with what finally worked, so that others might not have to repeat my experience. I eventually found a tandem (brake & clutch) master cylinder that would work. It is a Raybestos MC36347. This is not listed for my 1966 GMC HM5500 truck, but does fit and has the needed 1 1/2" brake bore and 1 1/8" clutch bore. The clutch slave I got is a Centric 138.80003. There are Bendix products that also appear to fit.
Unfortunately, it seems that both the Raybestos and Bendix master cylinders are now out of production. I got the Raybestos part from old stock being sold for a great price ($100 with free shipping) on Ebay. So don't delay if you think one of these will be needed in the future. I found the Centric clutch slave at an on-line Porche parts dealer. It seems to still be in production.
Then I found out that the master cylinder does not come with new dust boots, and the ones on the truck were completely rotted away. These fit against and through the firewall, rather than on the master cylinder, and were impossible for me to find. Eventually, I came across some workable replacements in the cardboard "boot box" at a nearby power brake and hydrovac rebuilding business.
After getting new steel and hose lines put together for the rotted out clutch lines (including some cutting and flaring of my own to re-use the custom bent line going into the clutch slave), I was able to put it all back together. Then I found out that air would not leave the clutch line with the usual push and hold the pedal and open the bleeder valve (many times) procedure. So I broke down and bought a Mityvac bleeder, which eventually worked after I put Teflon tape around the bleeder valve threads so there would be enough vacuum to pull fluid through the lines.
Both the brakes and clutch are finally working again, and I was able to drive (meaning move) the truck last week. In the process, I also found and repaired loose and leaking vacuum lines to the hydrovac and to the two-speed rear end switch, and there was a major vacuum leak where a hole had been burned through the manifold vacuum supply hose (at the back of the engine) following a previous owner's repair that pushed it against an exhaust manifold (so the truck also idles better now). I was doing this in a field during the California drought. Now it is raining (thankfully), and I will have to wait until the ground gets firm to drive the truck under a tree to remove the steel rack frames, which is finally back to what I wanted to do a month ago.
I hope this helps, or at least forewarns, a future shade tree mechanic's clutch and brake repairs on a mid-60's GMC medium duty truck.
