This will be an awesome build!! I'm not sure I completely understand the 800 series and 900 series cab configuration.
Moving the axles is no big deal just take a rubber hammer to knock off the rivets...
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Also what benefit is compound turbos? I am seeing 35pis or better from the stock turbo. Are y'all thinking that more boost is possible for the 8.3? I have a giant intercooler and I can't keep it cool. Granted it makes a whole lot of power but there is still more useable fuel on tap. I would not be opposed to adding a different turbo or building a combo setup. I'll be curious to see what you come up with.