Thanks for the lead. Searched and found:
http://www.laroke.com/larryk4674/smokey_culver/hubcaps.htmLooks like 4 is the magic number. I was going to rivet but looks like screws win.
So far this is sorted:
4 clips (via screws)
No spacers needed (I may use them because I have them JUST in case)
3600/3800 are different caps from 3100
Final question open:
Did you change lug studs to the larger diameter when running these rims?