I went the other way and did simple corner joints on SQ tube and then capped the open ends. There are a bunch of reasons why I preferred doing it this way.
1. Simple straight 90 degree cuts do not require changes to the cutoff saw settings and are most accurate to layout and make.
2. Welding along a pointy corner, even if ground back for proper penetration is not as easy as welding a seam on a face.
3. Jigging/clamping is easier. The sides and xmbr's can be pulled together w/o sliding past each other.
4. The radius of the long side tube makes a natural weld prep for good penetration with the cross tube, so two sides of the joint require less grinding prep.
5. If you hold the length of the long tube back enough to allow for a flush cap/closure plate, and make the closure plate slightly smaller than the cross section of the tube, it provides a nice "cabinet corner" fillet around three sides (that is easy to weld and comes out looking nice and rounded/chamfered); and leaves a nice weld prep for good penetration on the forth edge (the one against the xmbr).
Another reason I did it this way was because I wanted to round the front vertical edges of my main rails to match the radius on my wall profile. It was easier to cap the radius with a segment of round pipe that way than it would have been with mitered corners; but either way, the above reasons are enough that I would still have used the same joint style.
Don't forget to drill a small vent hole in any tubing that you are trying to weld shut. Otherwise the air inside the tube will expand from the heat of welding and will try to push out thru your molten weld puddle, making a nasty porous blowout like a volcano. With the dedicated vent hole(s) once you are done with all of the structural welds and everything has cooled off, you can do a quick weld, like a tack, to cap the vent hole(s), getting in and out fast before it has a chance to do the same.
My chassis construction started in earnest here:
TPCE Trailer Fab linkyScan down
this post to "Blocked the xmbr up off of the sleeve..." and you can see a little more detail about how I did my end caps on the small swivel coupler xmbr at the front of the tongue. (It happens to be mitered because of the tongue angle, but was still welded as a corner joint, so don't let that confuse you.)