[dsraven]

like rapid says, springs and shocks out, use some sort of link in place of the shocks.
in doing my frame swap, I installed links across the shock mounts before taking the body off the frame. the entire weight of the donor was sitting on the frame so the suspension was at ride height like it was designed for. then after the swap is all done I will measure rocker heights etc and remove the links from the shocks. then adjust springs to get the rode height correct again. my case is a little different in that my donor uses strut front suspension, so like a coil over. I can get an aftermarket adjustable coil over later. the rear suspension id coil spring but the vehicle did come from factory with available rear air bags. if I need to I can easily convert to air bags in the rear with off the shelf parts from GM or aftermarket.