Eric, I assume you have a Morse cable which is typical of Vintage Spyders?
My cable is mounted to the floor about 6 inches behind the gas pedal. It goes down the center and through the firewall, the under the engine curving up to the upper engine/trans bolt by the starter. No rubber mounts at all. I also have absolutely no vibration either. I drive barefoot in the summer so I'd definitely feel it.
I also have a solid nosecone mount and a trans strap, so it's not like I have everything rubber-mounted.