I did the same thing on my 67. I cut a square piece of sheetmetal, and from the inside bolted it in using the existing 4 holes from the heater motor. Painted it black to match the firewall. I then drilled a 1" hole and put a grommet in the middle and use that to route my A/F gauge wires through.

Here are some pictures before I did the grommet

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1967 Mustang Coupe
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