Paul,
I was out at the airport today and got the firewall measurements you asked
for.  I added station 19 because this is where the outer edge of the tunnel
is located (Your drawing shows this as station 18.)

Referring to your drawing, the dims are as follows:
    0 - 1/4"
    3 - 0"
    6 - 1/4"
    9 - .7"
  12 - 1.4"
  15 - 2.55"
  18 - 4.8"
  19 - 5.8"

A 5" radius would be about right for the tunnels.

The measurement on the front motor mounts is the outside-to-outside
dimension and is 20 1/8", best I can measure.

Thanks,
Mark

You lost me there Mark. Is this the dwg you are referring to?
If not send me the dwg.

Paul Lamar