From the looks of things, the only thing that separates the HD'* from the non'* is the diff and the housing. There may be other little things like fiber clutches (non-HD) vs. metallic clutches (HD). And I am pretty certain the torque converter between the two are different as well.
Another thing to look at too. The trannies that came in the Rivieras use a cast aluminum structural side pan versus stamped steel non-structural pans in our cars. Not too sure if this adds to the overal strength of the trans (I don't think it does...it is more for mounting to the subframe than anything), but it does look cool
Welcome, downwardly mobile yuppies, to the dawn of the Age of Nefarious.
1993 SSEi - FrankenJinx - dead and gone.
1994 SSE (136K strong) - 180* t-stat,HD alternator, most unequipped SSE on the club, XM radio, Kenwood CD/MP3.
2006 Chevy Malibu Maxx LTZ - Custom 2.25" manifold-back exhaust w/FM-80. Probably the only Maxx with this system.