I recently sold a nice 5 bolt short rolling frame for $300. It had NO rust damage at all, was straight and painted with POR15.
The rust out of a bed or cab support is not a big deal but, BUT, the rust that is between the rear supension crossmember where it is rivited to the frame rails IS! That stuff you see between the metal is the metal expanding do to rust. Most of the damage there you can't see as it is hidden by the surface metal. I would be careful with that as if that rust is fairly thick, then the frame rails are weak which can mean a problem waiting to happen. Removing and patching is a job that requires a good welder who knows what he is doing. Got a buddy with a 5bolt, lowered ($400 kit with control arms etc installed), SWB rolling frame that can be had for $400. Not sure on the rust but could find out----.
When you add up the cost of parts, shipping costs, work ---to get that rusted frame solid----you may actually exceed the cost of getting a decent rust free 5 bolt setup. Patience grasshopper----a better one may come along and you will feel better about the rest of the project knowing that you are starting with a solid platform. My .02
