First of all, compression tests are very misunderstood, and given far too much credence. These do far less to tell of an engine's health than they do to diagnose a problem in real time. The actual numbers you're seeing don't mean much. With respect to the numbers, the general rule of thumb is 75% of the input value; traditionally where 80 psi is used, this means you want to see at least 60 psi. However, what you really need to look for is the trend; just like oil analysis, you're looking for a number trend, not at the actual numbers themselves.
The differential compression numbers will vary with the test unit used. They will vary with the person doing the test. They will vary with the temperature of the engine. Often they'll vary if you do the test twice in a row. They'll vary significantly with the position of the propeller (and consequently the crankshaft, camshaft, piston, and valves in the cylinder).
What the test does first and foremost for you is allow you to see an obvious problem, and hear a leak through the inductionor exhaust. You'll hear all kinds of wild, incorrect ideas about why compression might be low (my favorite one is the notion that the piston ring gaps lined up will cause a low reading, which is utterly ridiculous). Ignore them.
Your compression has been consistant for the last three years. Be happy with that. It's not particularly high. So what? Are you seeing significant oil burn? Having vibration or power problems?
If you were to have cracks in the cylinder significant enough to cause compression loss, you wouldn't need a dye penetrant check to find them; you'd hear it. Trust me.
So far as large orfice vs. small; large bore orfices are meant for large engines, such as R-2600's, etc. Your 0-320 is a small engine. If you hold the prop steady at it's high compression point (and you can get an inaccurate, artificial reading by messing with the prop while you do the test, as you may have noted), and give it a moment, you won't be hindered by the orfice test. For small engines, Continental (not lycoming) requires a calibrated orfice.
I get the impression that you're not a mechanic, but for some reason you're doin the test on your own. I get this impression because you aren't very versed in what you're doing. I bring this up for two reasons; first, diagnosing on your own may not be your best bet, and second, if you're not careful that prop can really hurt you during the test.
Why are you referring to AC 43-13 instead of the manufacturer maintenance manual?