I run single drive shafts on 187 wheel base tractors with SPL250 drive shafts. But going to 3 drive shafts won't hurt anything. I do a lot of frame work. I wouldn't consider rigging an air ride. Buy yourself an air ride and throw away the original springs or get one off of an old school bus. This is a good one for a medium duty truck.

http://www.watsonsuspensions.com/Truck/Drive-Axles/DA-1700

You need to start right. Put the thing on jack stands and adjust it to where the frame is level. Then make sure what you add on is the same level. Also you need a straight edge to make sure the frame members are straight. If you not going to use the old suspension, I would just start with a bare frame rail. That way you would only have one splice instead of three. When you mock it up, pull a tape from a front U-Bolt and make an X to make the thing square. I have seen people create real nightmares when they just get out a torch and a welder.