IMHO get your pipe in there so you can get the recommended clearance from the tail cone to pipe. Then just bolt it pretty much where you have it. Shim it so you get the tail cone centering in the pipe inlet.

Here's a link showing my Jetcat P-100 Install. I think your Wren is lighter. It should make for an excellent powerplant.

http://www.rcuniverse.com/forum/rc-j...l#post11962537

As far as CG you can use your batteries, fuel pump and valves to do that.

Have Fun!