Unfortunately, mechanical design is a bit more complicated than someone giving you a simple formula to put numbers in. There are lots of formulas. The knowledge is in choosing and using them properly.
You can't choose the right formula with defining your problem. Like how much force, and in which direction, your structure will experience,
how far can it deflect, etc. I don't think you're going to get a "formula" from us unless you can completely define your problem in real engineering terms. The better your definition is the easier it is to choose the formulas (note the plural form here).
I suspect nearly all after market mechanical designs for cars is done by trial and error. The problem with this is the cost of the errors. In any case, whatever method you use, testing with the expectation that things might break is really important.
OTOH, I'll toss one out. Probably the single most important and useful formula in practical engineering: F = ma; Newton's 2nd law of motion.
BTW, I vote for
steel, which is what the rest of your truck's load bearing structures are made of.