For a given size, a square shape (or rectangular shape) has a higher torsional stiffness, but also weighs more. Looking at charts of tube properties, if you reduce the wall thickness of the square shape so that it matches the weight of the round shape, the torsional stiffness is very similar, with the square shape having a slight advantage.

I agree with your thinking of the round tube for the front beam, but would double check the wall thickness with the stock size to make sure you are not using a weaker section. I like the idea of the rectangular rear beam, but am a bit leary of trying to weld to it. I work in the metals industry and we try to avoid doing structural welds on aluminum. It is very likely that when doing the weld, you will heat the aluminum to the point where you will change the temper of it making it a bit more brittle than the rest of the tube. This could result in a stress concentration and ultimately a failure point.

How about using the rectanular tube and have some Stainless steel brackets or straps made for the bolt attachments (thru bolted to the tube). You could then re-shape the beam socket around the tube using epoxy and cab-o-sil or some other high density filler. I'm in no way an expert, but this is my 2 cents....

Good luck!!!

TB