[JohnJ@RideTech]

The air springs you are speaking of have a combined lift capacity of 7000lbs and are standard equipment on the HD 4-Link. There are actually multiple customers pulling large enclosed trailers loaded to the brim with these kits, the tow rig you saw in previous pictures was one of them.
What type of load are you planning on carrying with your truck once it is complete? While the lift capability and ride quality will be great, I am afraid that our kit isn't designed to lay frame right out of the box. If you are looking to go that low some modifications will need be made to the front cross member as it will hit the ground before the frame.
Putting a different rear differential under your truck shouldn't be a problem at all. The brackets are a weld on piece and may only require minimal modification around the axle tube to be installed.
Feel free to contact me directly if you have any further questions. Ill be more than happy to help.