Chevrolet didn't have a truck that they considered Heavy duty until 1956. That's when the Heavy duty line of 7000, 8000, 9000, & 10000 series trucks came out. On the '47-'55 1st series, & '55 2nd series (you left 1947 out) All 4000, 5000, & 6000 series trucks were considered Medium duty Trucks. 4100,& 4400 series trucks with a gvw of 14000 lbs. were considered 1 1/2 ton, 5100, 5400, & 5700 series COE's with 16000lb. gvw were considered 2 ton, & 6100, 6400, & 6500 series trucks with a 16000lb. gvw were 2 ton trucks. The second digit in the series designations represented the wheelbase. So to answer your question, a 6500 series truck was no more heavier duty than any other 6000, or 5000 series truck. Just had the longest wheelbase of the 6000 series trucks.
