Vintage Air & smooth firewall question

[Rod Run]

The big brown truck dropped off my system today. I got my condenser and everything outside installed, but then I ran into a roadblock.

I mounted all the brackets on my evaporator and now have a question.

The instructions say to use 2 bolts through holes in the firewall, to mount the rear bracket. My firewall is smoothed, so I have no holes. This means I have no way to mount the bracket, or even locate the evaporator. What did you all do, that have shaved firewalls?

Thank you