OK I put together the attached PDF since there are quite a few items going on here. A synopsis for those that don't want the gory details of the attached file.
1. There are two sizes of oval holes that were potentially plugged - a quantity of four on the radiator front apron, and a quantity of two on the inner frame rails near the idler arm and steering gear box.
2. For the front apron, there appears to be a running change at all three assembly plants from oval holes to square holes.
3. I did a quick survey of some unrestored cars, and there are some reasonable conclusions to make, although more examples would be nice to have to pin things down a bit more. I have more data myself that that will take time that I don't have for the next week or two.
Of course I'm still looking for sources of both size plugs. As Bob mentioned donor cars, eBay, and the like are probably the only way around this.
Also I did not have time to look in the assembly manuals, but with the attached pictures and dimensions that should be a relatively easy search. But the better half is dragging me out shopping. Yeah, I know.