You are probably right about a tractor being less expensive than a long framed straight truck. There is no reason that a frame cannot be strecthed and welded but you need to know what you are doing. Truck frames are made of high tensile material that is sometimes heat treated. This stuff can be welded successfully by the right people. You will want to make sure that the pieces you weld in are made of the correct material as well. Some folks choose to have frame rails made the slip inside of the existing frame and bolt to it. My toter was stretched this way. This method may be more practical for the guys building their own trucks. Make sure that you extension rails are made of good material and use the proper bolts. Double frames do have a small disadvantage. They are prone to rusting between the frame layers which can cause trouble in the long term.
1989 FLD120 with 2000 NRC conversion