My 70 cruise harness grommet also measures 3/4.

From my experiences with power window grommets, I think the sheetmetal hole will be 1/8 smaller to provide a super snug fit for the grommet.

I was hoping someone could chime in with some actual firewall measurements to verify this. Surely someone out there must have a 4 speed E body in mid-restoration with the grommet out so they can get an acurate measurement?

Tav