I understand the idea. But, the threads in the shafts are for extraction purposes. If they are joined face to face using those extraction threads, it might be a challenge to get them out later. Perhaps just buying the later model shafts would accomplish what you are after.
I've done the oring "solution" upon the recommendation of Mike Nixon. They are stuffed between the end plug and the outer shaft, applying lateral pressure to both. The theory is that the rockers will turn easily on the shaft and it doesn't take much to force that instead of the whole shaft turning.
I didn't have 10,000 miles on the one with that oring mod (and a brand new old style cover). And the bike is now in new hands. So, I can't say if practice lives up to theory.
As mentioned, the pressure point oring add on, shouldn't hurt anything. But, I'm very skeptical that it would have a miracle effect on an already worn valve cover, now out of proper tolerances to hold the shaft in proper geometry.
