I would have a question for you, and then suggestion on how to reach your goals:
Old guys like me that reached driving age in the malaise area of the 70s have a point of reference for
“ performance” that is wildly different for younger people from the computer/fuel injection generation.
A well tuned 383 roadrunner is a strong running car to someone like me yet my current daily driver turbo gas VW wagon (that gets 38mpg) would run neck and neck with that old Mopar. My point is if your point of reference of a fast car is something like a current coyote mustang/392 Challenger pretty much anything you do to “hop up” The old Polara is going to leave you disappointed.
Here’s a suggestion: before you start changing anything find a Way to drive or ride in a proper running/tuned old Mopar. A restored stock 383HP would be great on one end at the spectrum and then try on a worked over 440.
Honestly, you may be seriously disappointed with both, and that will answer your question whether to even spin a wrench at all under the hood of that old Polara. If you find you enjoy the experience comparing the two, stock 383HP (which you could easily duplicate for minimal cost) or a complete modified new build, Could be a fun future for you and your 65.
Steve weim55 Colorado