Dodge Dart 4-Door 1964 - Coming back to life

I finally got it out! Now I can begin the fun job; Removing all the rust in the enginebay! :)
And obviously on a 50 year old car.. Some people may have been there before me, there's alot of fibreglass bondo everywhere on the wheelhouses..
Why was this ever considered a legit repair method by anyone?

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Is there anything I need to think about once I start to disassemble the K-member ?