What you showed looks right to me, small bracket that holds back side of tank and a little of the bottom, the rest covered by strap. No extra crossmembers and not supported by any.
I don't have any pictures of my outside the frame 48 tank handy, other than what I already linked to.

•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-