round tube to cut down on the tires hitting the frame you are going to get by widening it, if you are running toyota axles since bigger tires hit the frame with it at stock width.
Sticking with leafs or going to links? that would play a big part in how much you could widen it with out causing a bunch of interference you would only have to worry about it closer to the firewall with links.
But if I were going to do it I would use cut every joint I was welding at an angle so there would be no vert welds, cover every joint with a fish mouth plate and add gussets where possible.
Im also guessing this is for the boxer. What are the dimensions compared to a 22re? Im guessing you are trying to keep the valve covers from getting scrubbed? or is it really tall enough to need to be placed further down in the frame rails?
$500 used 22re and you could be wheelin in less than a week.
Just saying. A foot on each side is ALOT.
If each head is going to be wider than a frame rail, where the shocks going to go? They move alot in and out when cycled.
If the shocks dont rub the tires, then they have been pushed back almost to the frame, which is not going to need that space.
Shoulda just flipped the diffs and put that can of worms in the back.