MJCronin,
I agree with you on the shape issue. I also have specified ASME VIII for tanks simply because it is a contractually neat way to define the quality and anticipate the resultant design. It also ensures suffciently robust nozzles or saddle (or even shell). Unfortunately the expensive features such as manways are grossly overdesigned.
By contrast, when faced with rectangular tanks, loose steel design manuals, Codes and "industry practice" are often specified, with the end result a mystery.
I have often used API 12F for shop fabricated tanks, but that generally implies vertical, flat bottomed tanks - typically oilfield "frac" tanks. Perhaps this standard could be the basis for a more generic tank standard?
Regards,
Bill