The conversion box is called Cable-X. LINK

You could also use a GPS converter but I'm not sure if they make one that hooks to an old school cable speedometer.

If you find an S10 tail housing with a cable connection (1988 and earlier S10, Sonoma and S15 trucks) then the tail housing could be swapped and the VSS reluctor wheel removed and replaced with a drive gear. If the location is different than the machined area on the main shaft, then a shim can be used to build up the thinner area of the main shaft so the drive gear can be mounted at the proper location.
Photos are of a T5 main shaft with VSS reluctor wheel.






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