Drive and driven gears: HD had 33/37, Non HD had 35/35 (this effects final gear ratio)
Output shaft: HD is about 1/8" or so longer than Non-HD. I've heard of any not swapping this w/o any issues.
Output Sun Gear: This gear is imperative to be matched to the differential. You can count the teeth and determine which differential is in the trans.
Differential: HD is a 3.29 ratio, non-HD are different and smaller in size as well.
Diff cover: HD is larger for the larger diff.
All other parts are the same in the same year/same version of the trans.
00 Regal GS: All kinds of cool stuff.