Mitch, for now my suggestion would be to buy a template as seen in this ebay auction
https://www.ebay.com/itm/37571813138...Bk9SR5Sw0Z_gZA
...and then keep my eyes peeled for an NOS GM rear antenna kit, which I don't think is an impossible find. Of course, the rear antenna option is coded on the cowl tag so maybe using original stuff is not critical but I like to use NOS stuff when I can and it's within reason. Just my 2 cents.