I would think carefully about using steel only. If your bed was designed to have a wood floor the cross members are say 18" apart? And if it were to have a thin steel top as you are looking at then I would guess somewhere around 8 to 12" is a better spacing for the cross members. If you set something heavy that lands between the cross members and has a point load, like a heavy desk, or a machine tool with feet, then the steel will not stay flat for long.

Do you have any pictures of the bed and how it is constructed?

I would recommend white oak as a nice looking, strong, long lasting, and reasonably cheep floor.

Grigg

•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-