Are you figuring on flat or semigloss? That can make a big difference in the finish. My suggestion is to get a small sample of the paint you intend to use, some Floetrol or Penetrol depending on acrylic vs. alkyd, and work out a bit on flat sheet metal.
I did tests for the outer sheet metal with a 3" fine foam roller and acrylic latex semigloss but found it set too fast and didn't have enough flow for my taste, whether I did it with a 'wet' or 'dry' roller. I was being super picky wanting it to lean toward a sprayed texture. Came out just short of that. In retrospect, it was probably very good with the wet roller. The 'dry' roller and lots of back 'n forth gave a CARC-like texture. If I was working with current 383 or tan flat I'd have loved it.
I did the inside of my bed with epoxy latex (satin) porch and floor with a 6" foam and liked the results a lot. It deadens out a lot with time. The bed is not as critical to me as the outside and the surfaces are already roughed up.
Both of the above had max amounts of Floetrol/gal.
Before the weather started to catch up, I was all set to do the same to recoat my tailgate, this time with industrial alkyd because it flows MUCH better. Still might have time to get to that. I'm anticipating very good results after using that paint with a chisel-tipped trim brush on my front pintle assembly. Virtually undiscernable brush tracks, even within 3'. And that result was without Penetrol.
Steve