P&M,
Blodgett and Megyesy do have a section on rectangular tank design, they are easy to follow and provide a good lesson in tank design. UL 142 also touches on rectangular tank design, but only slightly.
A resource I have used quite extensively for rectangular tank design is Roark's Formulas for Stress and Strain, by Warren C.Young. This is an excellent resource for many applications, rectangular and flat wall/roof tanks included. It will help you determine where you need bracing, but the design of the bracing will need to be completed with good engineering practice, or with the aid of Roark's or the AISC Manual, or other structural handbook.
Once you have your bracing locations known and sized, be sure the braces are seal welded together where they intersect, and should any of the braces intersect a fitting, taper the brace down into the fitting wall and weld the brace to the fitting. This keeps the stress path continuous and allows it to follow the brace into the anchors and into the floor/foundation. It will also eliminate stress risers in the area of the fitting.
Good luck,
Scott