Here is what I did to my 60's T-case. I tapped the original hole with a pipe thread tap and then screw the 90* piece into the case. I then screwed the barbed piece into the 90* piece. I will then run hose up into the engine bay.
Been looking into this since I was told a clogged breather can pressurize the tcase and cause it to push oil past the speedometer driven gear seals and, consequently, up through the speedo cable. Replacing the seals (and the gear if mine is torn up or heavily worn) is on the list, but I also plan on seeing what I can do about the breather. I have a little more room to work with than most since I have a 1" body lift courtesy Roger Brown (highly recommended BTW, along with the poly body mount kit) so I'm hoping I can get something figured out. Be nice to actually remedy the problem before replacing seals so it can be sure to not happen again (at least for a good long time)