Craig, my '73 challenger (w/ '74 rallye wiring) doesn't have a special hole for the grey tach wire, it goes thru the terminal block; it's the upper left wire (next to the purple one) in the connector closest to the brake booster. I know this is chall info, but I'd swear my '74 barracuda was identical.

In order to know what you want, you'll probably have to find out what you don't want.