I had to weld a couple of reinforcements to the plates as shown in the pics below.
So, you have a welder, _why_ didn't you do the whole thing yourself? I get it when guys simply don't have either the know-how or the equipment to do something, but that one just went right by me.
The hardest part of hacking up a stock pipe is welding the thin gauge mild steel shell back together. Mig welding it is probably the easiest way to do it, speed is the key if you want decent looking beads on it. As far as the inside part goes, 30-45 seconds with a sanding disc would probably make a huge difference in it not breaking loose. The braces you added will certainly help.