Don't be surprised if you don't get any Follow-ups to your question. What you're building is a retainer for FUEL TANKS.
I think this might scare a few 'follow-uppers' away. At the same time, there may be some BOCA code issues too since the retainers are, well, fuel-related. It might be best served to see how 'others' in your area built them or had them built. Other than that, your project is not going to move very 'fast' if you were to wait for Follow-ups here.
Maybe others will disagree. I wish I could offer more. My best to ya and hope this helps.