The cepek kit is prolly clarke's. In the rear to much a 4in. Kit an add a leaf and a shackle hanger flip will make it set level in the rear. I wouldn't worry about crazy flex it is really not needed in a truck like you are building. The add a leaf and hanger flip will work fine and will help with spring wrap. The rear ford springs flex pretty good in stock form.

I would also re think tire size. Your current axles will hold up nicely to 35's and you could still get away with stock gears. If going larger you are really taking chances and asking for breakage, and will require a gear change so that. Your truck won't be a dog around town and on the highway. Once again this is just my opinion that I have formed from my projects and experinces.