wehat do u mean
if the speedo is electronic and not cable all u need is the electronic speedo drive that comes factory in most 20v gboxs and run the signal wire to the electronic speedo
i dont quite understand what u wanna do
which speedo u using
if u run yr factory analoge one then just swap the speedo drive from yr orig box to the 20v box
tell me which speedo (in the dash) u wanna run and what speedo drive u have in the g/box