Another option you might consider is air assist bags. These are air bags that fit inside the coil spring and you fill just like air shocks.
This is a great way to go. You get the smoother ride that a 1/2 ton provides, but when you need temporary extra capacity you air up the coil assist bag. This also puts the extra load where it was designed to go, into the frame and the trailing arms. Unlike air shocks which attempt to hold up the load via the shock mounts, which were not designed to see these loads.
Here is a photo of an air spring in one of our era trucks:
http://inlinethumb30.webshots.com/47517/2104497230103882008S600x600Q85.jpgEmail the folks at Airlift Company, they make a living selling these and can steer you in the right direction.
http://www.airliftcompany.com/products/air-springs/air-springs-overview