After obtaining a pan and stripping it down in preparation for shortening, the first thing is to measure the clutch and accelerator tube protrusion and exit angles from the floor pan for later reference.
After a trial fit, measure for squareness, check level, and weld the two halves (Mig is best for this task).
To add strength to the sides of your body you may wish to use some thick wall 30 x30 mm RHS, either inside or underneath. Bend to required shape (cut out a v and re-weld or have it bent at fabrication shop) put a length of 20 x 10 mm flat inside the RHS, secure by drilling some holes in the RHS, then spot weld (through the holes) into position, cap both ends. Mark holes in RHS to coincide with mounting holes in floor pan/body, drill and tap. Apply some rust preventive into the RHS through the holes, done this way no water will enter the RHS and should last many years.