Mike B is correct. The '66 (at least this one) is vacuum accuated.
The entire cable, from the shifter lever housing to the valve, has been removed and is missing.
I can switch the axle by crawling under the truck and switching the 4 port valve where the cable would attach so, if I can locate a cable I'll be set. However, So far all the truck places around here are telling me it's NLA, "switch her over to electric".
I have one more place to try in OK about 65 miles north. I'm told they have a good number of old 2 ton chevys.
I just got the new engine running yesterday. Rebuilt SM420 is installed and a new brake, clutch master cylinder awaits installation.
She also received some new driveshaft carrier brngs. Now... got to find that cable.
