RE: Picco 45 DF
The Shrike may not be the right platform for that prop-engine combo, I'd go with something with less frontal area. A Demon, Diamond Dust, Matney Models Zoomer or some type of F3D planes. I think you may need a new header(or use spacers[:'(] )and pipe but try what you have. A pipe that's sized too small will be hard to tune and blow plugs you may need to get a bigger one. My tuned length is 9 1/2' from the center of piston to the large diameter on the pipe (apex).
You should use the pressure tap that I see on your pipe, check that its open and the header and pipe should be almost touching or the coupling will blow out in short order.