A compression tester will let you know if all eight cylinders are equal... and, that's about all it can tell you. Without taking the heads off--and I'm not suggesting that--you really have no way of knowing what you've got.
If those are 3418915 "J" heads, they're open chamber (and quite good) but you've no idea if/how much they've been milled and hence, the chamber volume. I'm not sure what's on the bottoms of the pistons, but if it's not stamped or hand-scribed, they're cast. That most likely means they're nothing particularly wild. The only meaningful markings on any of my forged pistons are stamped in the top.
Here's the trick, though: My 340 has pistons that are .026"
above the deck at TDC and (had) ~70cc heads, with a .027" thick gasket. I would bet you a crisp $20 bill it had
lower cranking compression than a stock '68 318 with its pistons .100" down the hole at TDC and ~58cc chambers. Why? The
camshaft. The solid roller cam in the 340 has oodles of overlap, a narrow lobe separation angle, and lifters that don't squish down. It doesn't
start to make torque until north of 4,000RPM. The '68 318 has a much higher LSA, far less overlap, and hydraulic lifters. All of those things keeps more air in the cylinders for a longer time. The 318 might have 160PSI cranking and my 340 145PSI (I've never measured).
There's nothing wrong with the 340; it's just how the camshaft affects
dynamic compression--the actual compression of a rotating engine, rather than the static measurement of volume @ TDC v. volume at BDC. The 318 is a full point
lower static compression than the 340 but higher in dynamic at compression-test RPM levels. At higher RPM, hydraulic lifters begin to act like solids and the 318's dynamic ratio drops too, getting closer to that of the 340. But as the RPM reaches the big end of the tachometer, those same factors (especially the cam) keep the 318 from moving enough air to hang with the 340's potential for RPM. The 318 can't even reach my mid-7,000RPM shift point, much less make any power up there.
Now that you got an unwanted lesson...
You can't calculate your compression ratio from a compression test, but it will tell you if the cylinders are equally
healthy. If they're all within about 10PSI, that's fine. A better way to compare actual health would be a leakdown test. If all eight cylinders had 130PSI or so, but a leakdown test of one cylinder showed 15% blowby, that would mean the engine's pretty worn. I wouldn't expect to see that if you think the engine's been overhauled.
As far as the valvetrain situation, let sleeping dogs lie below the level of the heads. Leave the cam and lifters alone, assuming you know each lifter is on its correct lobe. If you don't want the adjustable rockers, replace them with the ones
@mr340 gave you (after a good cleaning) and new stock-length pushrods for non-adjustable rockers and a hydraulic cam, preferably something a little beefier than stock. Just look 'em up for a '72 340 at your speed vendor of choice. You don't even have to remove the intake to do it, just be certain you get each pushrod centered in its lifter. I do it by feel, but you can see down there with a penlight if you need visual confirmation. DO NOT DROP A PUSHROD.
They can travel all the way to the oil pan given the right circumstance.
If the lifters got mixed up, you've gotta at least break 'em in again but you really should replace them. 99% of failed cams/lifters are due to improper break-in, so making sure everything's as right as possible before you twist the key is paramount. Timing, oiling, etc. If you have to shut down during the cam break-in, just go ahead and buy a new cam and lifters. It won't be OK, no matter how strongly you think or hope it will. If you have to do a new break-in, let me know
before posting that something went wrong and you had to shut it down ten minutes into the process. I'll give you a "pre-flight" checklist that'll make it go as smoothly as possible and reduce the chances of having to stop before it's fully heat-cycled.
