We have a 1935 Ford Coupe with a Chevy small block, Chevy 350 turbo transmission and a Desoto speedometer, which has never worked since we got the vehicle.  Is it possible to buy a universal speedometer cable that will hook up the two different makes?  If so, where can I get it?  If not...any ideas how to go about solving the problem?  

United Speedometer and others can make a new cable with the right ends.

http://www.speedometershop.com/