BTW, I like your choice of thinner 3x3 SFC, I have toyed with that solution vs say 3x2, but never was able to get a good/acceptable method of tie in at the rear, front is no issue of course. Any close up pics of yours in the rear. I suspect there are wide differences in rear frames in this area, my cars i could get it to work out.
Sure:
I did cut the tops off of the sections at the rear, then welded them closed again to maintain a fully boxed member from the Torsion bar X member to the rear rails. The 3x3 was .120 wall, I could have used .090 to save weight with no loss in strength. The metal supplier didn't have 3x3 in .090 wall though.
In the areas where the connectors are taller than the 3", I cut sections of .120 wall metal to match the floorpan. I seam sealed the edges for a cleaner look.
