Hmmm..... If my memory serves me correctly, the 3100 is a 1/2 ton model, I've never heard of a 6600, but there are many things I've never heard of. It sounds like a big truck, 2 ton, 21/2 ton..... larger maybe. If so, I think only the cab is the same as the 3100, everything else should be different (larger)

But..... since I don't know what a 6600 is, I could be wrong.