I'm wondering if anyone knows how much pressure this speedo fitting is under.  
Some of you may have been following my story about swapping in an oddball AMC transmission that's essentially a Ford 3.03 with a planetary gear OD setup on the back...

Anyway- I want to cap off this speedo (of course it's the spot that's the tightest location) but I don't know how much pressure this is under.  One guy using the same transmission said he used silicone to plug it up but I'm not sure I understood what he plugged up.  The threaded piece fits over the crimped nipple.  The guys on the HAMB said to put a rubber cap on it.  I'm not so sure that the rubber cap I have is stout enough, but it fits.  

Looking for thoughts on how much pressure this fitting is under...and how else I can get rid of it...Can I snip this crimped part off completely?  Should I remove the blue gear?  
Thanks in advance.