This discussion has of course prompted me to go back to pictures I took of my 67 Convertible during disassembly. This allowed me to get straight, head on shots of parts that otherwise would have been a challenge. The first two pictures clearly show the "slits" (with no optical illusion), which I have noted in the first picture by yellow arrows ; the dimples are also evident(once you stop looking at the crud and rust marks), and I have noted them with yellow circles. Note that I have also marked with two Red arrows, holes in the angled portion of the radiator support. This goes back to the early 80's when my Dad had the car, Even being a convertible (and actually even more so), driving in the St. Louis 90° + temperatures and 95% + humidity in July and August was pretty uncomfortable. My Dad said he wanted to add A/C. Of course it had to be "original Ford". Even back then, the 1967 Hang-on units were not easy to come by. I was lucky enough to not only find him an original 67 Hang-on Unit, but a NOS Installation Kit (brackets, hoses, fan blade, condenser, clutch, belts, hardware, etc.) that included everything except the compressor. I had moved to Omaha by then, so on one, or probably two or more, of our trips back to St. Louis, I got it all installed and running for him.
In looking at the pictures I have attached, I then wondered why I didn't use the cutout location we have been discussing. I then pulled the Installation Instructions (copy attached), and found out why. The Instructions for the Hang-on clearly show that the holes were to be cut where I cut them. I believe this is because of the difference in the Condensers. For some reason, I didn't complete the opening between the two circle cutouts, but obviously I didn't need to, and maybe I was thinking ahead, that sometime, "down the road", it would be easier to fill in two holes, than one big oval opening.
Note that the Instructions call for two , 1-1/4" holes two be cut where the dimples are (apparently there were dimples in both places for the Factory A/C and the Hang-on, although being over 35 years ago, I don't remember, and was not int documenting thing like that back then.
For your cutout, I would suggest just buying a good hole saw (check your measurement, but probably the same 1-1/4"). By good, I mean the heavy duty, hardened steel type, similar to the one in the attached pictures. These usually run from $10-$12 in the 1-1/4" size range, and are readily available at Hardware stores (Ace), Menards, Home Depot, Lowes, etc.They come in several different ways ; with mandrel, with arbor, and by themselves, where they thread onto a mandrel that you have, or purchase separately. I prefer the type that has an arbor attached with a set screw for a pilot bit (usually 1/4"), that you supply, and can move to other similar types of different sizes (several companies make these including Morse, Ace, Red Devil, etc.). Over 40 plus years, I have accumulated sizes from 1/2" to 2-34" (usually available in 1/8" increments, and some in 1/16). These have many uses other than just automotive (household, remodeling, woodworking, etc.) and you won't regret buying a good one. Of the probably dozen and a half I have, there is not one that I have used only once or twice.
Hope this helps.
Bob
