The wood allows the frame and bed to flex independently with out much squeaking or wearing of metal to metal.

If you weld them together the frame will not flex as it should, cracks will start to appear.
If you don't use wood, the steel will rub on the steel and wear and make noise.

The wood was, and still is, a good idea.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-