Hi Gordon,
I am not certain what you are asking.  It would seem you are asking if you can use a DIY engine mount, and therefore, you are asking how far from the firewall should you place your engine?  This is what I found with my engine.  I am using a 13B NA engine with a PowerSport redrive.  The weight of my engine and redrive is 295 lbs.  My radiator weighs 15 lbs and the oil cooler weights 7 lbs.  I found that when adding the weight of the radiator and oil cooler fluids, that my engine system would come very close to the weight of a Lycoming 360.  It was estimated that the average weight of the prop would be 27 lbs.  My Ivoprop weighs less than that, but the prop is positioned much farther anterior than with a Lycoming because of the redrive.   So, the arm is out there.  Tracy's point on the cg of the engine being positioned just inside the first end plate would be pretty accurate. 

I am getting closer to finishing a Zenith 801, so I am yet unable to offer actual flight experience.  Zenith did provide numbers for the installation of a Lycoming 360 and I compared my numbers to theirs.  I had anticipated that I would be placing my engine a great distance from the firewall because everyone says the 13B is so light.  However, after adding the weight of all the "extras," plus the fact that the arm of my prop is out a much greater distance than with a Lycoming, I actually had to move the engine closer to the firewall than if I used a Lycoming.  I concluded that I needed to design and build my own engine mount.

I hope this is helpful.

Ken johnson