Buddy,
That would be in the late 70's to early 80's. Early 90's produced an "SD" model, that was a class 4 15K gvwr or there abouts truck with a pickup body. GM also did a 3500HD from about 92 to about 2000 with a body lifted truck that also was a class 4 15K gvwr rig. Ford IMHO hit the nail with the currect body style 450/550 setup. GM bit the big one with the 4500/5500 setup being more of an mdt style cab. I could see that with the 5500 as it was a 19500-26K gvwr rig. the 4500 was inline with the 450/550 in the 14-19500 gvwr range depending upon axels, springs etc chosen. Then the 6500 was inline with the 5500 gvwr wise, but the 5500 only had the 6.6td option from a diesel standpoint, the 6500 had Cat and an Isuzu inline 6 option, BOTH had the 8.1 V8 gas option! BUT, one could get GCWR's into the mid 40K range with a 6500 vs 30K with the 5500. SO a better trailer pulling option. Hence showing why initially the 6.6 dmax was designed for rigs to 26K max from the get go. Be it good or bad frankly.
I would have gotten a 4500 in 05 "IF" GM had had more of a 25-35 series cab and chassis setup like the 450 did. The 450 is a good rig, I just do not like the ford cab as well as GM's. Now you are into more personal opinion that is oen truly better than the other......
marty