The wet vs dry compression checks were probably a means to check for compression ring wear on used engines. The wet tests would only be applicable IF it was known that all of the oil squirted into the cyl was EVENLY distributed around the complete circumference of the bore. To me, THAT would be a significant issue, in theory. Even with a lead-down test.
I know the engine was recently rebuilt, possibly to a less-intense level than can be done in modern times (deck plate honed, plateau honing, or similar), BUT I also suspect it might not be that much different than what would have been "the norm" when the car was a "used car". Back then, if there were no piston sounds, it was "rings, bearings, and a valve job", usually. BTAIM
Personally, I doubt the rabbit hole of compression/leak-down testing will not lead to a definitive answer. MANY engines with compression pressures as yours run smoothly at all rpm levels, by observation. No matter the rated compression ratio.
What would I do, myself? I would seek out a repair shop which had an ignition oscilloscope "machine" onto which to watch the traces for each cylinder as the points worked and each cylinder fired. Some could arranged the traces with each cylinder vertically, or in one horizontal "line". In this way, the complete ignition system's activities could be watched in real time at any rpm level. I suspect that that might now indicate "a miss" at higher rpm levels, though, so putting a higher load on the engine at lower rpm levels (as in driving up a hill and maintaining/increasing speed) might be needed. IF the vibration gets worse under load and lower rpm levels, that could indicate an ignition issue.
IF new pistons were installed, were they just installed OR were they "weight-matched" to the old pistons or against the OEM service manual specs for such? As a general rule, ONLY OEM replacement pistons (up to .030" oversize) weigh the same as the production pistons. I suspect that if you can find a catalog for the replacement pistons, you'll discover they are heavier than the OEM piston weight specs, which is normal.
In engine balancing, there is "over-balance" and "under-balance", depending upon the main rpm ranges the engine will operate in. I've only heard my late machine shop operative mention that, so I'm not fully-versed on what those terms mean. In this realm, worst-case scenario might be to pull the engine and get the rotating assy checked for "balance" to factory specs. Hopefully that will not be needed IF an ignition system issue can be discovered!
Carburetor? Usually an over-lean mixture will only happen at cruise conditions. More throttle activates the "power system" for a richer mixture and more power for passing or going up hills. Too lean can result in less power than normal and even lean spit-backs through the carburetor.
I recall some earlier concerns about "cam break-in time" in earlier posts. Many times, such mentions were NOT in the FSM, as I suspect those things should have been known by any competent mechanic, back then. It might also have been that with the less lift, less valve spring pressure, and such, that such initial run-in time was not needed, when it really was. Getting a sample of the motor oil (with a "kit" to pull oil out of the dipstick tube and capture several ounces in a bottle to send off for analysis) would pinpoint such an issue very precisely or quickly, typically. Probably might be done by a local CAT equipment dealer?
An alternative might be to remove the oil filter element (or cut it open if it is a spin-on) and look for "sparkles" between the pleats. Although the analysis would probably be more definitive, I suspect. Do both for a total better investigation, though.
So, you might go out and drive the car to get it fully warmed up with over 10 miles at highway speeds. Then if no hills, at a speed when the engine is smooth, start adding throttle to the point where the transmission kicks-down into "passing gear" (I seem to recall this being a PowerFlite car?). IF the engine runs smooth, but when passing gear happens, the vibrations start, that's probably an engine balance issue. Should the "missing" start before that, that can point to the ignition area, most probably. IF this last situation is operative, then holding the car to a slower speed with the foot brake and higher throttle should probably duplicate that at a slower road speed. OR even firmly applying the foot brake and throttling into the engine to put higher loads on it, to cause the ignition to "break down under load". Should the brakes not be up to that task, DO NOT DO IT that way!
A leak-down test can be good on a race motor to determine if it might need new rings for ultimate power production. On a stock motor, it can identify if the lower compression is due to poor ring seal of poor valve sealing, as each would sent "air" to either the intake manifold (intake valves), exhaust pipe (exhaust valves), or crankcase (rings), in general.
As you mentioned no lower-rpm running issues, I'm presuming the valves are good. There is a shade-tree method of checking for burnt exhaust valves, by feeling the exhaust pulses at the end of the tail pipe at idle, plus lower power on acceleration and a rough idle.
These are ALL things you can do "solo", by yourself, other than the ignition traces observation at the repair shop with that diagnostic tool.
Sorry for the length. Also sorry if some of what I mentioned might not be what you wanted to hear. Hopefully it will be something simple in the ignition system!
Take care,
CBODY67
