I got a chance to get a little work done Sunday on the pickup. There is no core support reproduced for the 65-66 2wd F100 pickups, but it doesn't take too long to make the 61-64 reproduction work.
The primary difference between the 61-64 and the 65-66 is the lack of support brackets for the inner fenders on the earlier trucks. Then, Ford changed the radiator mounts for 1966. Do what you want with the radiator support brackets. If they stay, buy a 65 radiator. Remove them and buy a 66 radiator.
First, to make it fit, you have to remove this bracket shown below:
It should be stubbier like this one:
If you are using this and want to keep the 65 style radiator mounts, you'll have to keep the original bracket and swap it over.
Then, I drilled out the spot welds holding the inner fender bracket to the original support. Usually, these are good enough to reuse. If it's really bad, it wouldn't take much to fabricate them out of some angled steel using the original as a pattern.
Next, I had to mock up the front end to find correct placement of the inner fenders:
I took an awl and scratched deep into the core support so that after removing the black primer from the support for welding, I'd still be able to find it.
I used a cupped wire wheel to clean off the primer and shot it with weld through primer.
Then, using the holes made for cleaning off the spot welds, I plug welded the brackets to the support.
You can see in that picture above that I have completely removed the 65 style radiator mount leaving holes where I drilled out the spot welds. I'll fill them in with the MIG for a finished final installation.
For my restoration, it's a lot better to replace a panel than to stitch a rusty old core support back together or pay a mint for a rust-free original only to find out it is rusty. I'm going to have the support blasted clean of the EDP to match the rest of the metal for finishing.