if i understood, you´re asking how to mount the engine support to have the muffler on the belly of the plane.
the (poorly) drawn sketch may be helpfull.
you trace center line and engine base on your firewall, then use the intersection point as a reference, make a 45 degree X, and replicate the engine support distance on the 45 degree lines, hope this helps
you may check if 45 degree is too much placing the engine over the newly drawn lines ok?