Okay fellas, well I was finally able to take some time and download some pics. Here is a few of an early "HD" case or better know as the th-400 "k" case. Like I said it's a little thicker, has a flywheel/starter cover thats made of the same aluminum/magnesium as the case. (Not just the regular stamped steel flywheel dust cover) As mentioned this is an early model case so it's stamped with 26 on top the later cases (I think he said 1980 and on) have the HD stamp. Otherwise it's identical to the regular 400. In some of the pics you can see the extra six bolt holes that run down the underside of the bell housing to attach the cover; on the underside of the cover you can see the two dogears. These two bolt hole locations are for two thick steel rods that attach to the cover and down the sides of the motor to the frame mounts. The rods which have plates on the end of them sandwich inbetween the framemounts and the motormounts. All they are for is to triangulate the drivetrain so you don't flex and break the bellhousing. Most likely I won't be able to utilize these bolt holes because a cars crossmember is in a different position than trucks are.
Like I said maybe I'm looking into this a little too hard but I figure if anything the case is a thicker, so "in theory" it should be a little stronger. And I like how the case has the cover which encloses the flywheel, starter bendix, and torque converter.