body mounts question

I'm in the process of a full body off restoration on a C3 (79).
I pulled the body of the chassis and now I see that the steel reinforcements on the number 1 and 4 mounts (on the body) are completly rusted away. They
sell these mounts new.
But how do you replace them? Especially the number 1 reinforcement seems a problem.
Thanks for any help

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