the inhibit switches are not named very well, they actually allow that gear to be used.
1st and Reverse should never go through the OD and DO NOT have inhibit switches - the torque could be too aggressive for it.
Like wise on the 6 cylinder engines 2nd gear is not used for the same reason.
What would be your aims if you could run 2nd OD - very limited use.
So if you have inhibit switches for 3rd and 4th then these need to be wired in parallel and then fed to earth/switch etc.
If you really want second OD then you will need an inhibit switch fitted to the GB top cover in the correct place and this goes in parallel with the other two.
If you have a similar switch for reverse then this could well be for Reverse lighting.
If you need to install a limit switch then the thread size to use is M16 x 2mm - however the actual thread is 5/8" x 13TPI but this does not exist.