1972 Chevrolet truck bed

I need to replace the floor in the bed of my 1972 chevrolet truck. The bed is definetly rotted out, it is also rusted up the bed wall approx 1 -2 inches. Does any one have experience repairing this? This is not a restoration or show truck at all. Its a work truck.
I know I must replace the metal, but what do you have brace or support while cutting out and welding in?

Any help would be greatly apreciated!