86 GMC fleet trucks (for a certain company) had 292 originally.They were retro-fitted with a 4.3 GM V-6 .The 4.3 was TB injected and had a computer.They had an adapter(inline)on the speedo cable for the computer to have a signal for proper operation of the updated system.
In your case, you have the sensor at the tranny where the old speedo gear was plugged in on the 85 and earlier trannys.You might try to find anearlier type tranny.It will be cheaper than going with an electronic speedo.

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