I'll apologize in advance for not answering your actual question, but if you are interested in alternatives,
I use a mechanical speedo drive from Dakota Digital for the EFI conversion in my '70 Pontiac. I wanted
to keep the original speedo working, and this unit has been great:
https://www.dakotadigital.com/pdf/ECD-100.pdf
Looks like it may have been superseded by a newer model though:
https://www.dakotadigital.com/pdf/ECD-200BT.pdf
They're a little spendy, so it really depends upon how much you want to keep your original dash
looking and working like stock.
-Scott
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