Since you recently purchased your Chevelle, check the wiring to the tach. I recently puchased a 68 Camaro SS and I have found a number of strange wiring situations.
Our muscle cars have been thru the mill with each owner changing things around. I recently noticed my amp gauge indicator bouncing around instead of a steady indication. I went through the wiring and found that the ground from the altinator attached to the water temperature sending unit on the block!
In my case, it looks like every Tom, Dick, and Harry messed with everything under the dash and under the hood!
