[jsbihn]

MrIgor,
Please understand that, though your question is genuine and I am sure you hope for an answer, most will probably question what you are doing. Remembering that the Hummer was designed to be a "Go Anywhere" type of "Off Road" truck/rig, and most will be using it for that and there might not be much experience with what you are asking. With that said (and the other things), we dont want to run you off because I am sure that everyone can contribute to the community and that is good thing.

To hopefully give you an answer to your question, I would say the following based on my experience building cars.
If you are looking to lower your vehicle, I would go with a true lowering kit that is made for your vehicle. The work is already done for you. Usually the parts are the same price and if for some reason they are cheaper, think of the time and energy you will have to put into it for the retro. Also from my experience, you will want to lower with springs.... not shocks. Springs carry the weight of the vehicle, not the shocks.... those are just used to control the dampening. (no shocks and the car/truck you are working on would pogo continuously). Now if you have done the springs all around and you need a shock with more dampening force/compression and a shorter throw to replace the stocks you have now, you can alway call Tokico, KYB, Bilstein, Fox (or who ever you choose) and ask them what will work. This way they can tell you what will work and what will fit. I figure better to put a little time into it and get it right then to just guess.
I just believe that when it comes to certain things, it is best to let those who know do the calculating for me.

Hope this helps