Been doing some research and most research is with someone changing out a TH 350 where the driveshaft does not have to be altered! If I do this conversion on the Cutlass is the driveshaft yoke compatible as I have a 400? And do I have to change the length? Any replies are greatly appreciated!

 

You would need a new driveshaft and yoke. On the plus side (I'm assuming the 1970 in your avatar) you won't need new parking brake cables as the 200-4R uses the 400 mount location.