Search for a chassis builder/fabricator in your area. That repair needs to be strong, not pretty. If there are any racers in your area they will know where to go for this type of fabrication and welding.

They will probably do the job if you thoroughly remove all carpet and interior pieces surrounding the repair area. They don't won't want to do that and you probably wouldn't want them doing it either, it's costly from a time perspective.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer