I don't have any actual experience trying to do what you ask, so these are just thoughts. As I'm sure you are aware Ford made major changes in the fullsize Ford ( Galaxie ) car starting in the 1965 model year, with the frame, suspension, steering, motor mounts, starter type, etc. all different. With that said, they are similar. I'm thinking about the original Ford 427 long cast iron exhaust manifolds, Ford had to make changes to the '65-'67 versions from the '63-'63 versions to make them fit around the upper control arms - the forward runners on both sides have to come back straighter before turning down. At first glace they seem pretty close, but on closer examination you see they are similar but different. I recently saw a Facebook post where the person showed they had a FE with the '65-'67 427 long cast iron exhaust manifolds, and they found their '63-'64 style FE road draft tube didn't fit either, the lower end of the later 427 long cast iron exhaust manifolds apparently also tuck in closer to the engine and differently, presumably due to chassis differences. And the original starters were different, although that may not be a consideration for you if you're running the newer starter, or an aftermarket one. The motor mount difference could be a concern too. My thought with all that said, is that there might be a better chance of, like you asked, '65 long tube headers fitting the '64 Galaxie then the other way around, since it seems like Ford had to shape the later 427 cast headers to fit a tighter space, but there is so much stuff different I'd hate to advise someone it would work without actually trying it. And headers are expensive to guess with.
On top of that, it probably depends on the brand of headers too - do you have a certain brand in mind?
Hopefully, maybe someone has some practical experience and will answer. It would be some good tech info to have.
