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

Yves