Frame swap?

tubular frame/chassis where you use round tube and connect  it all with a roll cage

or box tube frame where you use 2x3 square tube to build a perimeter frame.

To do it you will have to cut out all the floors and then build your frame inside of the rockers and trunk are and weld the body to the frame you build as the Early Torino's are unibody so the floors are not set up for the body to sit on top a frame.

You can buy back half sections for like when you narrow the rear and tub t with the frame rails already made with the bend for the rear axle.

Check Jeg's or Summit for Chassis kits.

I had a good friend in Florida who builds race car chassis's and that is what he does to ll the unibody car that are set up for drag racing.

with a 557 stroker that is the way I would go and get a Mustang II front suspension kit

bu you will have to build the chassis to the width you need then once the chassis is build with crossmembers  you can add some smaller 1 1/1 box tube for crossmember's  for sheet metal floor to attach to. I believe you can even buy a sheet metal trans tunnel kit already bent so all you will have to build is the flat parts of the floor with 18 gauge sheet metal.the race car guys use tine and the put alot of square tube structure under the seats and alone the trans tunnel to rivit the tin to but if you want to make it solid you can use the 18 gauge and weld it all in which will add strength