Ken,
The GMCs usually had the six clips and the Chevies had three clips. I was lucky enough to get four of each with my '54. I didn't want to blast the original paint off the better rims, so I used two Chevy and two GMC rims on the 'finished' truck.
Note that the clips were riveted on, not welded as far as I could tell. Are your rims still wearing the original paint? If so, the pinstripes are pretty distinctive - to help determine if they are original or not.
I put GM repro hubcaps on, and they fit the GMC rims just as well as they fit the Chevies. As for the caps, I believe that the 3100s came with EITHER 15" or 16" rims - I don't know what the 3600s came with - see if you can get the wheel size from the seller. The GM authorized repro hubcaps are pretty widely available if the 3600s are too big. Good luck.
