Look... Most Paint and Body Shops will
NEVER Perform the necessary High Liability work on what looks like
"A Crumb-Cake & Peanut Brittle - Accident Waiting To Happen" and Weld around 22 Years of Body Cancer caused by the collection of 22 Winters Worth of Brown, Salty Road Snow Slush doing such a Great Job at Dissolving Away at
Everything structural under there.
However... if it were *Mine To Keep* from the Salvage Yard Crusher... I would suggest that YOU place contact rulers along the Red Lines I super-imposed on your last Images... and Photo-Memorialize as many "On The Vehicle" Images with them of that "Metal Box" area as possible:
Then you'd very carefully have to use a Plasma Cutter and remove that Entire Rusty Box Section from under the SUV... Onto Your Bench ...and then From One End of it to the Other... You would need to Make a Stiff Paper or Thin Cardboard Model Copy Cut-Out of ALL of its Precise Dimensions and Landmark Relationships in all Three X, Y, Z Vectors.
But ...before doing anything else...
YOU SHOULD TAKE THE PAPER MODEL OF YOUR METAL TRAILING ARM BOX... BACK OUT TO THE SUV AND SEE WHETHER OR NOT IT ACTUALLY FITS IN PLACE INTO THE FRAME AND PROPERLY ALIGNS WITH ALL OF YOUR PREVIOUS FRAME MARKINGS...AND THEN SEE IF IT REALLY DOES FIT IN THERE...
Then it would be time to find the most appropriate Thickness and Dimensions of Each Piece of Mild Plate Steel necessary for your Pending Metal Cutting, Metal Shaping and Bench Welding Tasks.
You could either use that Plasma Cutter or a Gravity Band Saw to Shape them down to their exact sizes and Tack-Weld Each Segment into those identical Positions as compared with your Cardboard Model Segments... Making Dead Certain to
ONLY CUT OUT THE HOLES with a minimum of Two Relational Segments, Tack Welded together that Match the Identical Two Paper or Cardboard Cut-Outs that Fit PERFECTLY Together once they are properly oriented with each other ...BEFORE You Tack-Weld the Metal Plates together. Using several Right Angle Boxing Magnets can Help You Here.
In other words... While Working Very Carefully... While Working Very Gradually...You Would:
"Build That Metal Box To Scale... ON THE BENCH."
And once you are certain of the Fitments... Only THEN would You Drill Out the Holes that support the Trailing Arm Thru- Bolts... Then... after Anodizing that "Thing" in an Electrolytic Coating... Take it
BACK under the SUV...and Pre-Tack Weld it back under that Frame...Triple Checking each Prior Reference Point. Again the Boxing Magnets are your
BFF for this part.
Then... *IF*
EVERYTHING is Kosher... Stagger the Tack Welds about One Inch Apart, moving around the entire Box at opposite sides to avoid inducing any Warping or Shrinkage. Then...
Let It all Cool Down...and Measure it
"One Last Time...For Ol' Times Sake" BEFORE running the Final Staggered One Inch Long Welding Passes around the Perimeter of That Whole Damned
*THING*..
AND...
"Bob's Your Uncle!" :>)
...or you could just call around to your Local Welder's Supply and see if anyone they KNOW is looking for a Bench Top Welding Job like YOURS Requires... and be Ready to Pony Up That CASH! That Moolah! That *Elusive Spondulex*...You get the Idea... :>)