So rather than just close up the frame and patch shut the missing portions in the trunk floor, I would build a plate that would do both. PLUS I would incorporate GM's brace... but not out of 20 gauge sheet metal.
MY PLAN:
If welding the frame back shut with 1/8" steel plate builds strength back into the frame... making this plate twice as high and running it all the way to the front of the trunk would add MUCH more strength into the car.
Here the plan... the transparent red area is where I will be welding in a 1/8" steel plate to not only build back up the missing portion of the frame rail, but also add in a MUCH larger area for the inner wheelhouse to make it's connection to the frame and rear seat brace.
Here you can see how much more of the wheelhouse I will be making contact with and basically doubling the height of the frame rail in this section.
