The last picture was of a Heep like Pelagro's that was done a few years back. The hat channels were mostly salvaged originals. We had purchased a straight section, and spliced in what was rotten. The white oak that acted as a stiffener/filler holds in moisture, and causes the floor to rot. To keep the channel from collapsing when the hardware was torqued, there were thick wall sections of 1/2" i.d. pipe welded in. So before the channel was installed the sections were painted with several coats of the yellow industrial primer from TSC, then a couple of coats of the por 15, then a couple of coats of etching primer. The mating surfaces were coated with weld through primer to ensure that there would be no exposed metal.
To answer your question, I had a spare frame that the dimensions all matched the original frame. I used it as a jig to ensure that everything fit correctly when mated to the original. Everything was tacked, then rolled over on its side for ease of welding.
I used to have 5 gallon buckets filled with Cat hardware. When Caterpillar changed their logo from the packman C, to the pyramid up the A. They threw out boxes upon boxes of perfectly good, never used hardware. Everything I owned or worked on had Cat hardware.
I put a shrinking disk on my Christmas list, just behind the shrinker/ stretcher I need for making the bottoms of the tool compartment on the wagon.