You can use a 200R4, but only if you run less than 1000 hp. All overdrive automatics require a little massaging of the cross member and a new rear mount at a minimum. 518's and a lot of the others require major transmission tunnel work, the virtual removal of the center crossmember section and subsequent reinforcement and suck up lots more power to drive too. The gear ratios aren't as good either.
There's some good posts over in the Drivetrain section.
