1500HD is a 2500 for the most part with a semi floating axle, some 1500HDs are DEALER APPLIED LOGOS, factory 1500HDs are crew cab and 8 lug hubs
2500 has floater, but the regular frame and no real beef, frame taper to about a 4" cross section C channel under bed.
2500HD has the floater and the taller frame, 6" tall frame boxed section to channel under bed box, double plated thru the axle area ( PITA to drill thru ). HD frame was made taller to accomidate the Allison trans, which is only available behind the Dmax and the 8100
Allison 1000 trans is available in the 2500HD-C5500 trucks with the Dmax or 8100,
the "G"-van 2500-3500 cutaway bodies use 6.0/4L80e or the 8.1/4l85e.
Next month, the Dmax 6.6 will be released in the Gvan chassis, detuned using a 4L85E trans, gonna be a PITA to work on. Next year, same combo is "supposed" to be released in 'burbs and 'hoes, dunno about the Avalanche yet ( BTW, the 2500 Avalanche uses the 8.1 with 4L85E ).
points of interest ,
8100 is 496 cid,
502 engine is based off the 454 casting using siamesed cyl walls ( no coolant between ) 8100 is it's own line using jacketed bore. they are similar, but not the same.
GenIII 4.8/5.3/5.7/6.0 same general design, different bore and stroke, strong bottom end, cross bolted mains (must have a special fixture to reassemble correctly)
If yer interested in the 6.6 DuraMax, ask, I've been soaking in those all month.
Kevin