That is pretty odd, every one I have seen is pretty standard. Well, you could use some brass pipe reducers to bring it down to the right size. One should be available to go from what you have to what you need. Functionally it will be no different than just screwing in in. Any ACE Hardware should be able to fix you up in the plumbing section.
A brass reducer? Thanks again don... Here is picture .actually im looking at the axle breather spec and its 7/16-20.. So I need to go from that to 5/8 with some unkniwn thread...this sucks
I know you said 7/16-20 but, have you tried a 3/8 pipe to see if it will screw in and seal ? If so then you can use a 3/8 to 1/4 reducer bushing directly in the rearend breather hole and screw the 1/4 breather into that ?