The Factory Assembly Manual Section 8, Sheet 1.00 shows the 3102 Panel gas tank hung from two crossmembers spanning all the way across the frame and bolted to the top flange. But it sounds like you may have brackets for a 3602 (3/4 ton panel) that bolt just to the right side frame rail. Sheet 1.01 would be the installation diagram for that and it doesn't show any spacers and appears that the brackets are riveted to the frame.
