I'd go either way on the core support. You can easily trim the existing one (it essentially needs to become flush with the fender "face" surfaces) and use the donor for measurement if needed.
One thing I recently did was get a new grille in better shape and trim the lower edge to clear the big corners of the 78+ lower core support/frame area. 2 rectangles (one per side) taken out of the grille in the rows of slotted lower mounting holes.
We shot this whole thing just last week using my van for an episode of Hot Rod Garage (my friend is the host, has been aware of my project and pitched his producer for a quick single episode about it.) Also fixed the angle of the turn signals since that area of the fender (or is it part of the upper core support at that point?) is at a different angle.
Another tip: enlarge the bumper bolt holes on the lower core, the old bumper bracket spacing might be the same or close but whatever you get will probably be slightly tweaked and fighting some misaligned bracket bolts while holding up a bumper and trying not to kill yourself from frustration is pretty exhaustive/impossible. I also only ended up using the top bolts to hold the brackets on. Pretty sure you'll need new holes to use the lower ones. I haven't gotten around to that yet. Unsure if I will, it's solid enough as-is.