Your only actual question was answered in regards to the body lift and some additional input on the firewall/trans. tunnel was also thrown in.
What specifically is your plan? Body swap onto the new chassis or fitting the dmax into the old chassis? What transmission do you plan to use? I'll assume you are keeping it 4wd. You say its a half ton... does it have upgraded axles in the front and/or rear? i.e. D60 front and 14b rear? If not, and if you aren't using the new chassis, I'd highly recommend you at least get a 14b out back or better yet, fit the aam in there from the donor.
corporate 10 up front may last a while depending on your intended use. If you plan to make much power (and torque) AND USE IT REGULARLY, I'd find a D60 to put up front. Again that's assuming you aren't using the new chassis.
Speaking of front end... You'll need to be thinking about a transfer case. It's been done but it's not easy to mate a np205 to an allison directly. Some have ran them divorced. Obviously, I'm sure you're aware that the old GM front axles were pass. side drop whereas the nv261/263 are driver's side in the new trucks so you'll either have to rearrange the front axle (78-79 Ford HP D60 will practically bolt in place of a GM D60 and is driver's side drop) setup or find a different transfer case than the 261/263 already behind the allison.