Any gearbox breather discussion is a WAY over sold mod, even to the point of being ridiculous and should not be a real concern. Internet articles and forum postings on this subject are in my opinion of very low credibility. This is why.
I built a pressure test rig using a very low-pressure high-resolution gauge, regulator and ball valve I did a series of pressure tests of the AMC gearbox on my 72 combat. While the test included the demonstration of my clutch rod seals ability to restrain oil leakage, I was actually trying to debunk what I believed to be an unsubstantiated old wives tail about gearbox breathing/venting deficiency and the need for further breathing mods. This test was demonstrated at a few of the national rally tech sessions, and remained installed for an extended period of time to be able to redemonstrate the test at local tech sessions.
The first main leakage path is the path in the main shaft... this is what my CCRS fixes. A close second leakage path would be the normal early vent in the little round access cover or the MKIII brass tube style vent. The third path is the clutch cable entrance. This can NOT hold pressure, even if you seal around the cable with RTV, the leakage also goes up the cable itself up to the handle bar. A minor but variable fourth leakage path is the between the main shaft and sleeve gear bush.
In the final test the cable exit was totally plugged with a small rubber stopper and tested with soapy water for no leakage. Even with all the passages sealed, try as I might to actively pressurize the gearbox after totally sealing leakage paths one, two and three, there was no ability to view any residual pressure once the ball valve was shut to stop the regulated compressed air source. All remaining air pressure vented through the sleeve gear. The very high resolution...low pressure gauge went to zero quite quickly.........soapy water bubbles showed the air leaving. The results were as I expected, and the normal gearbox is absolutely NOT able to hold pressure, especially once the clutch cable is reinstalled.
I would have no problem using an undrilled MKIII access cover or without the MKIII brass vent on AMC gearboxes. The results of the test show that any theoretical air expansion causing a pressurization of the gearbox due to heat from operation is in my opinion totally bogus and without any merit what so ever.