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

 Paul,
 I can't locate the drawing you sent, but the "0" point was at the upper
 right motor mount point and it went down the side in 3" increments.  The
 19"
 point is at the outer lip of the exhaust tunnel.

 Mark

 Do you have MS Paint?

 Paul Lamar

Yes

Mark

OK load this into MS paint
and mark the points. Save and send back.

Paul Lamar