If you have a welder and the available tubing.
You could cut the hole and cut the brace and weld the brace to a pipe bigger than the exhaust tube to strengthen the frame.
Make sure to add holes to vent so the heat soak is not an issue.
Here is a poor SketchUp to try to explain.
Just an Idea
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The early bird gets the worm,
But the second mouse gets the cheese
84 Chevy K-20
63 Impala (my high school car)
http://67-72chevytrucks.com/vboard/s...Crew Cab Build