Believe all 356 engines from the 1940 introduction onward had hydraulic lifters so there is no adjustment as such. The plug gauge is used to verify the length of the valve stem when installing new valves or after grinding old ones that might have resulted in a lowering of the valves in the block enough to change clearance to the lifter. The same idea but a different dimension gauge is also used on later 288/327/359 hydraulic lifter engines.
A lifter could have failed or have a speck of dirt under the checkball causing a noise. The small curved pigtail tubing connecting the oil pump output port to the valve lifter gallery could have issues resulting in low oil flow to the lifters which would let them get noisy.
On the early 356 engines there was a valve which admits oil from the lifter gallery to the timing chain area which could fail resulting in low oil pressure to the lifters causing them to be noisy. That valve was changed in 42 production so should not be an issue on postwar Clippers.
If you need a gauge Pacific Northwest region reproduces them.
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