It's in the model number. The "210" is the larger displacement one, the "209" is the next size down. I can't remember what is below that but I think you'll want either 209 or 210 for onboard air.
I've had one for several years as one of my "someday" projects. It will be going on my F350 with the air horn from my grandfathers old truck.
They are broken down into "left and right" as well. If memory serves, it'll be an L or R after the model number (like 210R or 210L) and its the outlet direction. I "think" you'll want a left outlet if you lay it down on the drivers side and right for passenger side. If you keep it up right it shouldn't matter as much as long as you can route the lines. It's been a while since I looked it all up.
