Square and rectangular tube is almost always done as round tube that is
run through a 'turk's head" roller (4 rollers at 90 degrees to each
other and mounted at an angle to the direction of travel) I don't think
I've ever seen a DOM tube done as a square.
1/4" wall will be done to A500 (A, B, or C) specs, not A513 specs. Most
mills will have much better actual product than the specs call for. Call
your local supplier ask what they have.
I'd suggest that you will have better luck looking for even inch sizes
in this relatively large tube.