1968 Dart GTS Convertible project

If it was me, I would put the manual transmission back in. The fender tag should be able to tell you exactly what you have. The closer you stay to the original equipment the better the value of your car. Numbers matching is a huge part, then date code correct. I don't think that it's a good idea to add or take away from the original build of the car especially if value is a concern. Good luck with your car.