One of the reasons this is so interesting is that alot of the '66 Dart race cars were that exact combo. Post cars with 4-speeds. I've seen pics of Trans Am type cars that were built from that platform. (tell that to the T/A-AAR crowd :-D ) I've got a ton of questions... do you have the original shifter setup, I doubt 3.90 was the original gear, but is an 8.75 original to the car? The early cars had grips in the 7.25. Toolman and I both have original 8.75 cars which are rare. I know a guy who just parted out a '66 GT, 4-speed car that had the 7.25 grip. I have that cars info...I should just start a registry. Was your car a bench seat? (that would be AWESOME...bench seat 4-speed!) Does your car have the original disc brake master cylinder? What size wheels were factory, I'm guessing you would have had 14 inch...that is rare as well.... lots of questions. That is a rare and historically significant car.