Dumb question, sorry, but I am missing my Spedo cable to firewall rubber seal/grommet. I can't find one listed in the catalogs and the grommet isn't included with a new Spedometer cable for a '64-66 V8 w/C-4.
Well, it wouldn't be too difficult to fabricate one. The originals are very crude looking and covered with undercoat as well. That same gloppy undercoat/sealer that they shot all over the firewall (and the blower motor...) had pretty well obscured the seal on both of my 66s. You really couldn't tell what was sealing the hole up until you pulled the cable out.
If you were to take a strip of closed-cell foam (black would be best), wrap it around the cable, glue the ends together, and stuff it in the hole, I doubt anyone would be the wiser. Follow up with some of that undercoat and I supect that even a Gold Card judge (sorry Charles and Jeff *G*) couldn't tell the difference.