I too , have 2"x3" rectangular tubing sub-frame connectors . My approach " butterfly cut on three sides where it goes onto the crossmember . this gives you more weld surface area . Scribe the connector to the contour of the rear longitudinal . You will need to make a pie cut somewhere in the connector to make it fit . Maybe about 1/4" or so and weld it back together . some floor cutting is necessary but it's minor . I suggest TIG welding everything for maximum strength . Mine can't be seen unless you get down to ground level . Works great , looks great and cost $20 . winner winner chicken dinner
