OK Daniel,
I actually lucked out a bit...still have an original upper Radiator Grommet.
The Hole Size is approximately 1.25" but I would start slightly smaller; taking away material is far easier than putting it back.
The Grommets are available in sets, 378264-S, through NPD, (National Parts Depot), and other suppliers I'm sure, or you may be able to find good used originals or NOS Grommets. There is a difference between the two in terms of the inner hole sizes in each Grommet. The upper Grommet has a 9/16" hole the lower has a 3/8" hole; this applies for the Grommets at the firewall as well and are the same as the Radiator Grommets. The sets from suppliers include one of each size needed.
I don't have any info on the exact locations where the Grommets go through the firewall. If you have an original firewall pad that's intact, there are punch outs in the pad that will help locate where the holes should be punched through the firewall. If not, hopefully someone else has the dimensions.
Ray