Yeah, my motor was hitting my firewall too when I left the gbox in the factory position
So what did I do? I flattened the firewall to suit.
Then when I got the motor down a bit lower, I realised that the sump didn't clear the xmember, so I had to move the motor and gbox forward 25mm, on suddenly the firewall was not a problem, so I had to hammer it out back to factory position.
Luckily my shifter still come up through the hole (just), and I do plan to lengthen the tailshaft by 25mm in the future, just not yet.
But in my case, moving the motor and gbox was definately the easiest solution