PHANTOM2:

Ducting dimensions, both intake and exhaust, are governed by the fan's physical size, the power has no bearing. At least until your fan goes supersonic. With a 5" fan your limiting speed is right at 40,000 rpm, where the blade tips go over Mach. All sorts of strange things happen to the air flow when you get those shock waves bouncing around, up to and including total disintegration.

If you ever wondered what the intake doors were for on a supersonic plane, they limit the intake air flow to ensure the air velocity into the compressor is subsonic.

In other words, use you engine of choice, but keep the rpm below 40K with a 5" fan.

And just an additional note: your 5" fan, with a 12" pitch, has an air velocity well over 300 mph at 35K rpm. And a 10" pitch is right at 300 mph at the same rpm. I'm sure you can see that restriction of the flow, whether from being too small or a rough surface, will make for big losses.

Bill.