Welcome 65 to the club,
Spray- check me on this as I'm not 100% sure but didn't they zinc dip the Fords in the 60's? I know the 65 Thunderbird I just did was zinc'ed and I had to epoxy seal it.
This will also give you more of a build for your primer surfacer. unfortunatly sanding is the only way to achive the prep 65 and you will sand a bunch more I'm afraid. I prime & then block it down with 180 grit to check for high & low spots, and then prime again (3 coats each time), cross cut this prime coat with a soft pad, and then prime again for the wet sand with 400.
With all this body work you've done I would then seal it with a QUALITY sealer (I like Crossfire ) and let it set for a couple days.
Wet sand it with 600 & paint & clear.
Good Luck & let us know how it comes out.