There was another thread on options for replacing the tach, and I posted a picture of a tach from a Ford F-150 1980-86. I went to junkyard today, and saw one of that style, also shown below, but I also found one from a 1987 Ford truck. Someone had already pulled the instrument cluster from the truck too, so it was no work to get it out. Cost of the tach = $5.99 plus tax.
Anyway, it looks like that may be a great option for replacing all those non-functioning tachs. The second picture shows it side by side with the Eagle tach, and the 3rd photo shows it at the back of the instrument cluster. Needs to be trimmed to fit inside but that should be easy.
1986 Ford instrument cluster with tach (somewhat rectangular)
1987 Ford tach (left) and Eagle tach (right)
It will need some trimming to fit in but it looks like a good fit