I know that earlier GM tachs could be set for 4,6,or8 cyl by turning a setting on the back of it, I found this out on a Corvair Corsa that I converted to a mid-engine 350 with a Crown conversion they showed in there instructions how to set the tach from 4cyl. to 8cyl . I thought that mybe your tach could be set on the wrong setting like the 6cyl setting . Its a shot in the dark, but its possible. Good luck.