Here is a project that we are currently working on at the Hot Rod Institute. Its a 56 Chevy Nomad that is getting a transplant from a 2003 Z06 Vette. Currently myself and another student(the owner of the Nomad) have been building a full frame for it.
Actually believe it or not the floor wont have to be modified that much, but it will need a new tunnel and a small kick up in the rear to accomodate the new rear end setup.
Just curious about the portion between the front and rear wheels. A stock tri five frame does not kick like yours. The floor cross braces and body mounts will have to be reworked.
The frame is actually 58 inches wide on the outside, and the middle body mount will actually sit right on top of the new frame and then off of that we will make a shorter body mount for the front and the rear. The actual mounting points of the body is about 3 inches higher than the bottom of the rockers as is the entire floor pan underneath the car.