This is an OS mount for the OS 1.20 pumper, it gets the engine back as far as it will go, at this point moving it back doesn't do much for weight transfer. As for the fuel filler, it doesn't mater, the OS pumper has an open vent system anyway, a little more isn't going to hurt it. I did a search and found an old build thread posted in 2011 by Larger Scale88. Two of my old friends were doing the same build so they are both doing the thread. On page 6 it is mentioned that the distance from the fire wall to the thrust washer is 6.25 inches. From page 6 on, the thread is worth reading and there are a lot of photos. Several of them on how they mounted the servos aft and I think Jim ended up with his battery way back there too. Engines are mentioned a lot too but what I found was no one seemed to have a CG problem.
Go ahead and type in Great Planes Extra 300 in the search engine and read the thread from about page 6 on, it should be of help. I don't know how to post the URL or bring up old threads on RCU but it is several pages back on the search when it comes up.
I think it will be a big help to you.