Drilling firewall for engine mount
Your thread title made me raise an eyebrow...!
I had exactly the same concern as you and in the end its really not a problem. THe mismatch arises because the mount is welded up on a mould, but once it cools and is taken off the mould it distorts and hence no longer exactly matches the holes in the firewall.
You can move the holes slightly to take up some of the mismatch. IIRC we marked the holes in the desired positions using the best available fit, used a small round file to elongate the holes into the rough final position and then final drilled to get the holes round and the correct size.
You might well still have some mismatch but should just be able to "spring" the frame into place with your fingers. Vans advice was that there was no problem so long as I wasn't using a car jack, big crowbar, or other means of exacting excessive force, and that after 10 hours or so of engine running the fit would be perfect.
I have since been told by an A and P with warbird experience that getting the mounts for the Merlin engines in eg Spitfires and P51s often entails the use of a very large hammer and lots of brute force.
Chris