About the metal, I'll go with 1/4" on the extension as it's a diesel. For the plates i was thinking either 1/8 or 3/16. Not sure which one I should go with there. Was also thinking I should box the back where Ford/Dana decided to leave them open.

Thinking about it more, I am not sure if it effects the geometry the way I initially believed, but here's my geometric reasoning. I am going to be extending the arm behind the spring perch, effectively making the spring shorter, and creating less lift by the cosine of the angle the axle/beam makes with vertical. That in effect should make for more negative camber. At ten degrees that makes for less than half an inch of ride height difference (I ride height is probably closer to five degrees). The longer spring I chose should add a little more than half an inch so I suspect that I will be ballpark in the right spot for the turn that the kit was designed for. The one thing I didn't remember to measure today is the beam angle at estimated droop and ride height. Going to have to measure that tomorrow.

I also decided I am going to just do a 2" extension. Might as well stick with a round number. In the future I am going to be adding an axle which is 3" wider in the rear so if everything works out as planned, I will have to add 1/4" wheel spacers to the rear and everything will sit even. I know that math doesn't add up, but that's because of the increased angle of the beams at ride height it draws the tires in by about a quarter inch. I'll let a computer measure that difference before I buy some shim style wheel spacers though. Either that or I will just not care about the difference if it's close enough to not tell.