Grit,
Their were some 400 & 800 GMs badged with 1500HD that for ALL intents, were 8600 gvwr 2500's. Wolves in sheep's clothing.
The 400 HD 1500's you are referring to, in 88 they were 2500's, the 1500's had a 6600 gvwr, and the 3500's an 8609 gvw. No dullies until 90-91. 89 maybe 90 an 8600 2500 came out. Only engine option was a 350. The 454 was in a 9200 sw3500. When fully 3500 came into play, the 8 lug 2500/3500 and DW 3500 got the 350/454 options, and 6.2/6.5 diesel options. About this time iirc the 6 lug 2500 became a 1500. So one could get a 6600 or 7200 gvw version. The 7200 version is really no different than what you and I drive today.
Please note I'm going by memory. From owning an 88 K3500, 89 R3500 square body chassy cab, a 96 K3500 crew cab, 2000 reg cab C2500.
Marty