cut it out , it is spot welded in and a tar  seal is put in, If you can leave a lip then spot weld in a new piece if yours is as bad as this one I'm working on \
then you may have to go up on the sides and spot weld those in 
 This transitional 56 I'm doing right now , It had a poor restoration done , and now I had to replace the bottom corners and the panand the shelf in the main door    The person that did this packed in the bondo even in the bottom corners  .
It was in bad shape 

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