People do what you are saying but only when they want a special engine that isn't manufactured. They typically spend 100's of hours and gobs of money doing it. There is a lot involved. If you just want a berk of no particular prototype, considering the amount of work involved and the money you will spend on additional parts, the most practical choice is to buy a good running postwar berk with a beat up boiler, dressing it, and repainting it. You can get donors cheap.
The boilers are castings which are very tricky to weld. They can be brazed but the entire shell must be evenly preheated to the brazing temp. Otherwise they crack from expansion in a confined area. The same applies to welding but at a much higher pre-heat temp and the filler rod must be the same alloy as the shell alloy. If not, the joint will separate from dissimilar metal fatigue. Because stress is hardly an issue, pinning the casting pieces and cold joining them with super glue or JB weld is most practical. The frames are typically sheet metal but the motor assemblies must be completely disassembled before welding them together. Being thin, you can stitch weld but the slightest warpage will effect the mechanicals. Gas shield for sure here as you don't want spatter. Riveting them with splice plate would be more practical. Then you got the side rods to deal with. You can buy berk sets but will the wheel centerlines be the same? Plus where the rods and valve gear meet the cylinders are another issue.
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