I have a 1972 Cutlass Supeme with 455/TH400 setup with stock 10 bolt rearend. I do alot of highway driving and would like to find an overdrive transmission that would bolt up right to my set up to bring down the revs significantly. I thought about taller gears but I still want the acceleration.

 

The 200-4r overdrive would be your best bet. They will bolt right up to your engine and the crossmember stays in about the same place. You will have to get a longer driveshaft with TH350/700-R4/200-4R yolk. You will also have to do modification to your shift linkage. For my console shift, I went to shiftworks.com and bought their kit to make the stock shifter work with the overdrive. If it is a column shift you can usually adapt that yourself. To hookup the lock-up electronics and TV/detent cable you will probably need to leave that up to the person that builds transmissions.