http://rd-eng.com/pagetwo.html
R & D engineering has a mechanical speedo kit for a T-56 for $499 which is more than I paid for my tranny parts originally. The ZF is the better buy as the Mechanical speedo VSS is $75 or less if you go with the NAPA part listed in this thread. On the downside the Hurst ZF 6 speed shifter IS DISCONTINUED. I've been chasing a NOS one around America trying all the Corvette vendors. I'm not sure if one is coming to my house now or not. I got a shipping notification that was cryptic from ZIP Corvettes. They had one listed on their website as "in stock" and now that has been removed. Maybe I got the last one and maybe not. I may wind up with a B & M shifter by default.
Larry