I have a '69 Fastback with a 428 Super Cobra Jet, close ratio 4 speed, 4:11 Detroit Locker and power steering. That's the way Henry sold it and that is the way it will stay. Just a whole lot of fun to drive. Your car would be a lot of fun with a 390GT, they turn somewhere around 300 hp stock. A 289 with a few goodies would be fun too and that would be an easy swap. The stock 6 would be the easiest and keep the car original leaving you with the most value. Whatever you decide to do, Do what YOU want to do, it's your car. Have fun!