I did have to cut and modify the torsion bar cross member, but the floor remained untouched. They are limited to about 700 hp once properly built though, so I'm going to have to switch over to a 4L80 for my upcoming build.
Why not go with the Extreme automatics Stage III 200-4R? It's supposedly good for 1000hp/750ft-lbs torque. I know it isn't cheap but it's an easier swap than the 4L80 from what I understand.