Speedo cable with Columbia overdrive

[1stford]

Hey Fellas,

I have bumped up against an issue on my 1941 mercury convertible. The speedometer cable recently broke, it was the end at the transmission that snapped off. I ordered a new cable and it worked about 10 miles before it broke again. The issue appears to be that cables are too long which is keeping it from being in a relatively straight line so as the car moves the actual cable is trying to spin instead of just the inner part spinning, the cable has to have some loops in it to get to the transmission because of the 71 inch length.

The set up is a cable is coming out of the dash and into what I will call it a speedo junction box. Then the cable runs out the bottom of that and down to transmission. I'm not certain what that speedometer junction box that is mounted to the fire wall does but it seems I need a cable designed for that box VS a "standard" one. Hoping someone has experience with this. I have attached a picture of the piece mounted to the firewall.