Brackets are welded to the frame for the cab mounts, and with the new cab mounts and bolts, the cab is bolted to the frame. Once the cab is bolted in place, the floor and the bottom of the firewall can be welded together, and all the floor bracing and the back of the cab bracing can be added.
Pic 1, The cab is bolted to the frame.
Pic 2, The bottom of the firewall gets welded to the floor pan.
Pic 3, The support for the back of the cab at the gas tank cut out in position.
Pic 4, Cardboard is your friend! When you need to make something out of steel, but your not quite sure how to do it, its a lot cheaper to cut up cardboard (think beverage container box) its about the same thickness as the steel, and ridged enough to bend and sort of hold form. You can tape it together to make shapes, or to add more material. then if you really screw it up, you toss it and try again. its cheaper then steel to cut up and throw away.

One more for tonight.