Whole different beast than the avg. 2500.
Suspension lift kits and such that fit a 2500 will not fit a 2500HD.
The 2500HD could be had at one time with the 8100 Vortec, 8.1L, 496 ci..
The 8.1 is no longer available.
HD's also came with the Allison Tranny.
If I'm not mistaken, the 2500HD platform is basically a H2 drivetrain (motor, tranny, transfer case, diffs) with the HD pick up frame and sheet metal.
But I'm still not 100% positive on these "facts".