Not sure what you mean by bowing. Rocket Engineer has made it clear (since 2008 ) that no matter what size of stand is built, 2x4's are sufficient support for the weight (and that by a huge margin...think 10,000+ lbs. per leg.) The only real question is in the height of the top rails 2x4, 2x6, 2x8 etc. and deflection over a given span as well as the ability to prevent the stand from wracking. Honestly, as much as I would like a 7' open bottom stand, for my application because I am limited to a 34" high stand I might need to just go with 2x4 all around and deal with the 2 supports in front of the sump. I think 2x8 will eat up too much valuable space and limit my sump options. :-(0.018"
But at this point I personally start to consider the "legs" and bowing..
Should be fine though.
Different subject just for reference: