Ahhh, you're stripping it down bare and starting fresh. That makes more sense for fitting the 39s. So then you'll just be welding on leaf mounting points wherever they end up, eh?
I've never seen that particular bed before. I like it! Simple and effective with what looks like a functional platform up top for camping gear. (although a couple down bars for that roll-bar might be better) I'm sticking with my original bed for now (mainly because I don't have my own bender or worthwhile welder yet), but if it dies a horrible trail-death, I might have found a couple ideas to steal for a new bed. Careful about welding to the "frame". It's only sheetmetal after all.
What suspension were you contemplating for the front? Leaf? Longarm?
Jeep on!
--Pete