I'ma firm believer that the rail thru the pan welded fully is the stiffest. I cut the bottom out of the rear rail. Drilled the spot welds holding the remaining rail to the floor pan at the front. Then slid the 2x3 into the rear rail "sides". Clamped the the sides to the tube . Then welded it all back up thru the previously drilled spot weld holes. Plug welded the rail sides as well. .This pulls the frame rail sides in about 1/8" but everything fits nice and tight. Cut the slices in the floor narrow and work your way up to size.
Doug