John,
I called the folks at K&N Air Filters to properly size my filter.  It passes by a few percetage points.
They may be able to advise you on intake dimensions tho that is only a guess.  The engineer I spoke to was very knowlageable and maybe he can help you.  Get your info organized before you make the call.
He didn't seem too interested in the intake just assuming it was a ram (NACA) inlet.  Mostly, he wanted to know T.O. RPM, cubic inches, # of cylinders and if it was a turbocharged engine.  I just assumed he knew the ram intake pressure was adequate for supply purposes.
Let us know if you find out otherwise, please.
Walter P  N990Y  IV-P