Just tried to put a  new speedö cable in my 67 . A Scott Drake it was about 6"longer and the little brass collar on the speedo end was crimped such that it would not fit into the speedometer. I really do need that curse control.

How are you going to seal that cable?  Factory, had a sponge.  That is a big source of water leakage.  You could see the path of corrosion from that cable hole downward on my floorboard.  I have a suggestion on what to seal it with.  Body putty is too runny obviously.  Some have probably used silicone.   Here is what Im going to use.

http://www.lowes.com/pd_13591-1781-DS-110_0__?productId=4595233

They use this stuff in the heating and cooling industry to seal lots of areas.  They use it inside, outside.  I think this stuff will make a good seal, yet allow it to be moved and redone should the need ever arise.  

Ive also thought of rounding the hole and using a rubber plug with a hole in the middle.  

Greg B, let us know how the Lowe's stuff works out.