I owned a '63 years ago, red with red leather interior. One of the nicest drivers I ever owned. 260 V8 auto. I drove it for several years, then gave it to my dad and he drove it for another 6 years until he passed away.

I'm all about leaving the exterior of the car alone except for paint, then doing modifications to the suspension, steering,
brakes and drivetrain.

I'd be keeping my eyes open for a wrecked late model Mustang with a 5.0, 5-speed if your car is a 3-speed or 5.0/auto if your car is auto. Buy the whole car and use all the drivetrain including differential, cooling system, computer, wiring and all else that you need from it, then sell the remainder on ebay.