Not sure where/how to start:

[Capt. Pinbacker]

So i'm wanting to do the area between the framerails and the body on the truck (do the hardest thing first), but don't really know where or how to start:

Let's look at a few pics:





1) Should the fairing in the second picture sit flush with the bottom of the framerail or is it ok in this position?

Any other suggestions/questions?

[California98Civic]

Not sure what you are asking exactly, but it looks like you're thinking bellypan when you might consider side skirts and get more benefit:

[UltArc]

Nice images of the inner side skirts. For an underbody, I would advise including the lowest points of the vehicle. Also, I would advise starting with the front.

Starting with the rear (and allowing air to get in, aka, not stopping air from getting into it), will work like a bit of a parachute.