I think a lifter is stuck because it was stood idle for many months. It had a whole new head, cam and lifters less than 2k miles ago
 
Maybe, but diagnostics would then show significant side-to-side differences in o2 & fuel trim readings.

Grab a cheap dial gauge on eBay, and measure the cam lobe lift before doing all the camshaft out work. Might save a bunch of work removing plenum bananas, manifold, rockers, lifters, water pump, front cover, camchain, radiator(s), etc.
I’m going to just open up the rocker cover and not delve too much furthe. Looks a relatively straight forward job on a pushrod engine. I think the lifter will be stuck not worn so am going to replace without replacing the cam. The cam is fairly new
 

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