Speedo correction

I remember reading that there are a few outfits that make "converters" (for lack of a better term) that will correct what the speedometer is reading if the wheel size has been changed, or a different rear has been installed. My question is, does anyone know if these converters are only available to screw onto the speedometer itself, or are there also models that will attach to the tranny with the cable then attaching to the converter ?

https://www.dicksspeedotach.com/speedometer-adapters/

Most are made to be at the trans end with a 7/8 thread. The only one I know of that goes on the speedo end is the vacuum two speed ratio box. I suppose if the thread is correct you could mount the ratio box there, but it would be "backwards" and have to be built so.