Then on to the much needed valve job. I'd plugged in my vacuum gauge and it was holding at around 17" but was flickering much like scenario #3 on this site. However the description isn't correct (it's not a high powered engine). A valve job on a slant 6 needs the engine fully warmed up and is done while it's running. I popped off the rocker cover to reveal this gunky mess.
However closer inspection showed that #4 intake valve rocker had jumped off the pushrod! No wonder it was sounding a bit tappety and lacked power from a standing start. It was a small miracle that it was idling at all!
Loosened off the adjuster and reinstalled the pushrod and set about adjusting the valves. 0.010" for intake and 0.020" for exhaust. Feels weird at first putting a wrench on a moving rocker but after a while it becomes pretty easy. Several trips down the rocker shaft checking and adjusting and rechecking each one and it's now much quieter. A new rocker cover gasket and we are back in business and the annoying hesistation from a standing start has all but disappeared. Need to replace the vacuum hoses to the PCV/ carb and the choke pull off but it's running MUCH better.