My Freightliner toter was built on a class 8 chassis which was stretched. The P.O. removed the front driver and cut the frame. He had channels formed that would fit tightly inside of the original rails. He overlapped the channels on each end and bolted them together. There was no welding on this stretch job. Of course you can weld your frame. Welding is perfectly acceptable and done everyday. The frame sections are often cut on and angle and welded together. If done correctly, a fishplate may not be necessary. Remember, truck frames are made of high tensile steel and may be prone to cracking if you don't weld it correctly. You need to know what the material is and know what you are doing before welding a truck frame. The folks with the Antique Truck Historical Society have a message board and there are many discussions about frame welding there.
http://www.aths.org/