I think the first time I ran into the problem with dimpling ribs that get too tight to dimple near the aft end, I used the pop rivet dimpler. However, after the nail broke, I could not find a suitable replacement that would work with my cheap pop rivet tool.
After that, I used the "bend it a little method". You simply bend the flanges out a little so you can get a squeezer in the hole, then you squeeze the dimple. When you are done, you bend things back before riveting.
Ok, ok, before you all jump on me and say "you'll fatigue the part and it will crack", I'll just say that you don't bend the flanges that much, and you only do it once, so fatigue really should not be a problem. Just don't bend the parts back and forth. Also, having said that, I think drilling a countersunk hole in a thin plate is probably a better solution. I just didn't have any plates available when I did this.
Tracy.