A difference of 2 inches (50.8mm) is a huge difference. If you want the airplane to perform as well as the ads suggest you really want the engine mounted as close to spec as possible.
It seems to me that maybe its time to do a little kit bashing so you can mount the engine in the proper location.
Why does the fuel tank have to protrude thru the firewall? Is it really necessary?
Cheers.
James