It's likely a lite-ply firewall, so just drill your holes
with the recommended bit (and in the recommended spot).
Push the blind nuts just enough so they stay in place,
and then tighten the bolts: this will pull the blind nuts
tight to the firewall. A little dab of medium CA or 5 min
epoxy around the edges of the nuts will keep them from
shifting later. Don't be tempted to try some kind of
hammer operation !!

Have Fun, FrankB