goirish,
That does look like a Nitro models P-51. I'm building one for a buddy, the Lou IV, and it has the scribe lines on the firewall.

I thought if you looked inside at the back of the firewall, you would see the lines. of course that won't help much anyway.[:@]

The paper trick is the way to go with the cowl cutout, makes it foolproof.

I would mount the engine inverted, as long as your engine is well broken in, much cleaner look.

Just make sure your fuel tank is not too high.