I decided that a reasonable way to represent the 20" I-beam frame on 3156 was to make an I-beam from three pieces of 1/8" steel. I cut the strips on the band saw, they will be fastened together with 2-56 screws then silver soldered. This method should not result in any distortion which would be likely if they were welded.
I stumbled across an article in Modeltec, May 1992, page 24, where T. Parkinson describes cutting bar stock to simulate the I-beam frame of 3156. He used 1018 cold rolled steel. A fly cutter with a carbide insert was used to take heavy cuts and hollow out the material, then the edges of the beam were made by angling the milling head 12 degrees. He describes the finished product as "terrific". One bar on which they took all the cuts from one side before turning it over had to be straightened. I have since contacted T. Parkinson and found that the drawings and frame material are now with someone else. I have emailed this person to see how far along his 3156 is. No response yet.
Material has been rough cut from 1/8" steel and steel angle has been acquired. A lot of drilling and riveting will have to be done.