Overdrive tranny in an A-body
Overdrive, fuel injection and switching to big bolt pattern axles were the best mods I've made to my 68 A-Body. They are expensive and require some work, but the transformation to the performance, usability and fun is well worth it. Once the hood is shut and you are driving down the road, how you added these items is of little concern....until something breaks.
I went with a stroked 5.9 Magnum based motor, a reflashed factory JTEK computer with Hotwire Auto's Hot Rod harness, and an Extreme Automatics Stage 2 200R4 with a Precision of New Hampton 3800 rpm lock-up torque converter, an SFI rated Reid Bell housing (the plate style adapter is just not the best way to go) and 3.91 rear end gears.
There are many ways to upgrade to the big 3 mods mentioned above. To each his own.
I did have to trim and reinforce the torsion bar cross member and make my own transmission cross-member. The 200R4 has nearly perfect gearing, is more efficiently, more compact and stronger (once modified) than either a 500 or 518. I did not have to cut the floor at all.
The Gear Vendors just doesn't have a steep enough overdrive ratio for my tastes. I tried it in a buddies car for the weekend. It was just to little of a good thing for me, and I didn't enjoy having to try and split the gears while the car was launching forward.