I would avoid cutting the factory grommet. Could you relocate the alarm system wires (cut & splice) and plug the open hole with the existing grommet / plastic flex tubing housing the cruise vac lines & wires (assuming this is the hole in question). The pic shows a small hole drilled about 3/4" below the large grommet. There is a nice flat spot there to go through the firewall. Use this new hole for the alarm wires. This pic is of the IAT routing wires going to the ECM during my 84/85 - 86/87 ECM conversion. Sealed up with a small grommet.