[mr48chev]

Even with two leaves left and good bushings I would think that you will get more lateral motion than you want to deal with if you really drive the truck more than just posing around town.

With what I see in my mind that you are saying you want to do I'd just drop the I beam idea and go independent and be done with it. There are lots of proven and safe ways to do that so you can build a truck that will go as low as you want or even to low while being capable of being driven at any speed it has power to drive at with relative safety as far as the handling and predictability of what it is going to do when. 70+ on the freeway with a cobbled together airbagged I beam setup might be creating a situation where the truck wants to change lanes with a road surface change without giving notice that it is going to do it.