US Car Tool Core Support Stiffener
1) persuade the core support to be shaped more like the stiffener-
Might work some, but, not likely. It's anchored at the corners, so you'd have to stretch it. It will look like hammered dog crap when you're done, but at least it will be under the radiator.
2) persuade the stiffener to be shaped more like the core support-
Not gonna happen. You will only succeed in making it look like crap.
3) Ignore it and let there be a gap in the corner-
you'll see the wisdom in this
4) None of the above-
You could remove the original core support all together if it bugs you that much
I just did the install on my '71 GT. Other than that bottom leading edge, everything lined up great. Looks like I need to take some "finished" pictures of it. Here's the mock up.