Welding on frames is frowned upon and even pronounced "verboten" by GM in several tech flyers. But, given that the tail section is not really subjected to the kind of loading as the central portion, I expect it can be gotten away with as long as the weight applied to the extension is minimal.
But personally, I went with with bolt ons just to be safe. And, I also had to spread the distance between the rails while I was at it in order to make room for a place to hang a spare tire. Here's a couple of pix...
C-Channel frame extensions
The actual deck frame will rest on top of these and also bolt in place. That way if some nitwit (such as myself) ever damages part of the deck it will be a lot easier to repair than if all welded.
I have all the steel cut for the deck frame and will likely start assembling it fairly soon. The only weight it will carry will be a little Honda genny and, just maybe, a small dirt bike. Still thinking that one through.