I ran into the problem LockDoc mentioned about the incorrectly made reproduction rear tin. I ended up modifying my original tin to accept the tach. See posts 50-51 of the following thread:
https://67-72chevytrucks.com/vboard/...=813854&page=2
I uploaded a pattern for making the modifications here:
https://67-72chevytrucks.com/vboard/...1&postcount=56