Which 3rd member is correct for my car?

In most cases you would find a 741 case with a 3:23 SG unit as standard equiptment, with the 3:91 being the other option most chosen. However, because your car is a big block car, it is possible that your car could have had a 742 case. I will check with my buddy to see whats in his car, his is an all original 68 383S convertible. It's a 4 speed though, so it is not the tell all, and maybe there are others here with more info. Geof