340 Static Compression Numbers and Test Procedure

Well, if the engine was out, You could measure how much fluid it takes to fill one chamber with the piston at TDCcompression and the spark-plug hole as the high point. It's kindof hard tho after the engine is installed,lol.

But I call bs on the tester.
To get to an average of say 205psi with a typical big-cam Ica of say 67* would require a 12.5Scr.
By 60* the requirement is down to 11.6;and by 53* down to 11.1
The typical street cam will fall into the Ica range of 53* to 67*, which is just about 4 cam sizes; which covers from say 288 to 260. So my guess is the tester is off or the cam is just too small for the Static compression ratio. This can happen when a guy decams a race-motor for street.
If you redo the test, just keep cranking with the throttle at WOT, until you get two consecutive shots at about the same pressure, irregardless of how many shots it takes. Cranking speed will affect the readings, so when the last one is done, repeat the first one to verify it's still cranking right.
But like the guys say, this will be impossible to cure, perhaps even with thicker head gaskets and a different cam. So since it has to come apart anyway, I would just bite the bullet and take a head off. That ,or measure the cam, which means the front has to come off. Sooner or later the cam will have to be measured anyway,so it makes sense to me to just start there.

unless maybe it has Rhoads-type lifters in it.........