50' GMC 5 window, my first build

[49 CHEVY]

Hey everyone, I have started my first full build a few weeks ago. I picked up a fairly solid GMC half ton from PA after nagging the owner to sell it to me for months. It's a Colorado truck, killer patina, 5 window, all original.
Since its arrival to me, I've got the motor and trans pulled, front suspension stripped, and most of the unnecessary original parts torn off. Next step (after buying a welder), is to get my mustang II front end. my ultimate plan is to do full airride, 5.3 motor backed by a T56, and clean up the interior.
I will have many questions along the way and appreciate everyone's input in advance. My first set of questions is going to involve how to build a completely new rear frame section from the rear of the cab back. I want to use 2x6 square tubing and make a custom step notch. Does anyone have measurements for something like this? Also wondering what thickness of tubing I will need.
Here are some pics of the progress so far. Please be patient with my build thread, as I am saving for parts as I go and will not have new parts arriving at my door step daily.
Also, can someone school me on how to add more than 1 picture at once using my iPad? It's not working I well for me haha