Lennart,
The tuned pipe dimensions for MTW RE3 (in stock )are on my web page. A lot depends on how you configure the header.
I ran the engine with 350mm header length ( 14" ), Response was crisp and typical for a header that is too long, which is what is best for aerobatics. With that setup, I had 11 hp at 7100 rpm. These horses mainly are available for accelerating the prop.
The way you bend that 350mm into shape of course defines your total length. Best solution would be to leave a lot of tunnel length available behind the pipe so you can experiment, and cover it later with clear cote for aerodynamics.