Wow, would love to see a video and pics when you get it all done, sounds like it's gonna be a monster...
I've seen on some race cars where they run a turbo set up like that, pretty slick if I do say so.
Yeah I would deffently upgrade away from a chain driven case, but I would even go stronger then the 205, I'm not exactly sure your options on beefin that up, but I wouldn't imagine that would hold that much TQ even with good stuff in it, and and extra 150lbs or so will make up for it in strength....so if your going to take the time to put 2.5 ton axles in it, why not just throw a 2.5ton or 5 ton case in it that way your U-joints are the same size and you can run proportional drive shafts.....just my opinion, but it couldn't hurt to 'try' if you already got it, at the least you got a good expensive fire cracker.
But if you don't have the axles or t-case yet, your best bet is lookin for a complete truck, that way you get the whole set up...shafts, t-case, axles and all the little odds and ends and you can also so how it was set up and get ideas from that.....chances are if someone removed the axles they know how much they are worth and your going to pay what they are worth and not get all the extra goodies....with some luck, you can come across a farmer that just wants a 'non-running truck' off his property....
I agree with ndeepoffroad, just put her together, let her eat, and see what brakes, It's a toy/mud truck so your going to brake stuff and it's all a game to fix the week links and figure out whats going to brake next. I've broken plenty of 1/2, 3/4ton and 1ton axles in my time (hence my user name is what I named my truck). I've had all the combos under my truck and so far the 2.5ton had served me well, but lets not forget your you got twice the motor then me and it all depends how your going to drive your truck and what terrain your driving. So I think starting with the big axles first vs. makeing small incremental upgrades like I did over the years, with the rockwells your already ahead of the game and as with anything some adjustment and fine tuning will be needed, but nothing a blue wrench, some duct tape, and a welder can't fix.