I used some creative clamping to pull the drive shaft tunnel to the partial hoops and placed the C channels alongside to see how they'll look. I tacked the hoops in place. Enough for today; I'll finish the welds later.
The 3/16 inch pieces will be doubled up in a lap joint to form a partial hoop in the drive shaft tunnel. I traced the shape of the tunnel and used the shrinker and stretcher to match the curve. I checked the curve after tacking the pieces together, then completed the welds.
I need to brace the floor across the cab and decided to use some 16 gauge sheet metal formed into a C channel. My son's brake handled most of the pieces with 3/4 inch sides, but the 3/16 inch sides did not come out straight. I had to finish those with a hammer and dolly.