In case anyone else looks.
FAA-H-8083-1 weight and balance, down load free, shows a good diagram on page 3-6. It is the center of the axle regardless of any angle of the LG. I was supposed to know this since i r an AnP, just been a while.
Sullivan makes an anti siphon valve. Have one ordered.
Now looking for measurements to install a super smoke probe. Also watch out curved tip goes to rear of plane Dreamworks has a good diagram on orientation but no dimensions on where to position tip in exhaust.
Cheers
Charles