Re: Can a bad tach make an engine run poorly???
There's your problem. lol
Also, since it's an HEI, it does not need to use that wire, that is a ballast resistor wire, it knocks down the voltage for a points style ignition, splicing it won't change the voltage.
Run a wire from the fuse box that has power when the key is in the accessory position and when starting, easy to locate with a test light.
Also, as others said, 9 times out of 10 when you slide the clip in, it goes in between or on the sides of the posts they need to go on, been there done that, my tach jumped around also, and because when the shop replaced the body mounts, they ripped out the ground wires.
The only Mustang I'd ever own is a Fender.
'68 333" Camaro
'14 5.0 Mustang