Framerails as air tank?

I was planning on using my frame rails as an air tank but the guy who is welding them is a little concerned that pressure on a flat surface may cause a problem. I understand why air tanks are round and that flat surfaces are not conducive to this sort of application. I'm not to worried about the 2x4 .250 square tubing I'm using but I have some notch pieces that will be welded together that will be part of the frame rail. They are made from .250 steel and are 3.5 inches high, when I weld the top and bottom piece to them it will make them match up the the 2x4.

What do you guys think? Or where would I find info on figuring whether this would work, I can't think of how to go about figuring it.

Here is a pic of my notch piece. Basically a rounded 8 piece notch kit.

I don't think it would be a problem, I have seen lots of people do it. .250 wall is probably twice as thick as airtanks are made out of, and what about the pancake takes they have a pretty flat top to them.

I would do it without thinking twice but wtf do I know.

I have heard of a lot of hardcore 4 wheelers using their rollcages as air tanks to store for use on the trails, but they are round tubing, I would think being 1/4" thick it would not be an issue at all.

i think the thickness should be fine... we built a sq tank and unfortunately it buckled and bulged but found out the thickness the a hole gave us was wrong.... it was going into a bagged 64 linc....