Well there is another solution which I considered at one point as I had prior speedo problems as well (ended up replacing the entire speedo) and that would be an
electronic speedo - gaugeguys have one listed on their classic Smiths cobra gauges page
http://www.gaugeguys.com/Smiths/Authentic/cobra.htm which would mean seal off the mechanical connection port on the Tremec - no more leaks! But with a new speedo, new cable, no more leaks and no jumping speedo needle
for now I'm gonna stay with my mechanical gauge. I did change to Smiths voltmeter and electronic
oil pressure gauges for other reasons, and they are working great
