The HD stands for Heavy Duty and was used in the supercharged cars. Basically the only differences where a larger more durable torque converter and a larger differential to withstand the extra power and handle more stressful driving conditions. The HD diffs came in a 3.29 ratio and will interchange with a non-HD differential by changing a few other parts to make it work. Most of the gear changes in these transmissions are made by means of a different drive and driven chain sprocket arrangement. The stock HD transmissions, except the Comp G performance package, all had a 2.93 overall ratio and this was accomplished with a 37 drive and 33 driven sprocket setup with a 3.29 final drive/ differential. 37/33 = 1.121 x 3.29 = 2.93 . The GT or most all NA non-HD 3800 had a 35/35 setup which = 1.00 x 3.29 giving you a 3.29 overall ratio. I hope this helps you out and answers your question about the differences. There are other ratios available but are non HD final drives and the 3.29 is the most common
