The main difference in the models is the wheelbase. The 6100 had 130" wheelbase, the 6400 a 154" wheelbase and the 6500 was longest with a 172" wheelbase. The 6000 series were all 2 tons, some had different GVWs. The 6500 series were more likely to have the "jobmaster" 261 engines although my 4400 Napco came with a 261 so I'm sure there were exceptions.
