I second Kevin. Cut out the brace. I was fighting a similar dent on a hood for a 70. This is what I did. I took my spot weld cutter and removed the front bracing on the hood. This allowed me to get behind and slowly work the dent nearly back to shape. I unfortunately sold that unfinished hood as a replacement fell into my lap for next to nothing, and the new one had next to nothing for steel work on it.
This would be my recommendation:
Cut spot welds
Remove brace.
Hammer/dolly/ pull the dent.
Check frequently with a contour gauge.
Metal finish.
Epoxy prime where you can’t reach.
Weld brace back in.
More work? Yes. Easier to do? Perhaps. Easier to control mistakes? 100%. And no welding on the damaged area with little warpage at all if your dollying is pretty good.
That’s what I’d do, but I’ve never looked at the backside of a 64-66 hood before. I have one photo of that green hood completed sitting on my buddy’s 72 suburban.
Check the link for in progress photos.
https://photos.google.com/share/AF1...U0NXU3V1ZlBEd3Q3d3NPMW1FYi1INnoteUZKWEt3