Are you going to spray what you can reach or do a tear down to bare chassis? Has a bearing on what is suggested.
From your pictures your problem appears to be rusting where the factory did a crappy job of the original paint job in hard to reach places.
#1 answer is to have the frame and all chassis components powder coated whatever color you want. Problem solved. Assuming you are just doing a touch-up, painting whatever you can reach on the assembled car, you are down to a not perfect solution whatever you choose. You can cosmetically finish the spots you can see but I guarantee there are parts you can't see that will continue to rust.
If it were my truck, I would sand the rusty spots to firm metal, coat those spots with a rust converter such as Coroseal
(there are a bunch of these products out there), then sand, epoxy prime, and catalyzed urethane color coat/clear coat the whole frame. That is vastly superior to the just-barely factory job and will stop the problem where you can see it.