Like Paul, I made a female dimple die by countersinking a piece of 3/16 steel plate, which fits inside the narrow end of the rib. For the male die I've just used a rivet and gunned it with a mushroom set, but the dimple was not great. I get better results with an actual male dimple die in a rivet gun adapter shank, but there's a risk of snapping the nose off of the die if you don't hold it quite square.
For the riveting, there's a "transfer bar" way of back-riveting, using a bit of steel as a class 3 lever (mine's another bit of 3/16 about 1x3, with one end thinned for clearance). With the work piece face-down on the back rivet plate, one end of the transfer bar sits on the rivet tail, the other end rests on the plate, and the rivet gun smacks the middle of the bar. Bit of a pain...it needs more air, the bar likes to walk away, and you have to be careful about marring the rib web with the end of the bar. But it works.
(Edit: EAA has videos on this, tight quarter
dimpling and
riveting)
Or...yeah, MK-319-BS pulled rivets. I've done that too. Verify it with Vans for your application, but they gave me a blanket approval for substituting a 426-3 where the grip length is adequate.