How far you can mount your tank away from the engine/fire wall also depends on the engine. I have a few that are mounted several inches away from the fire wall and work just fine. Some engines and mufflers draw fuel and make pressure better then others, as close to the fire wall is still always a good idea. I also have several planes with the tank mounted more then a foot away from the engine and over the CG but these are all engines with some sort of pumping system like YS or OS pumper engines. A photo showing one of my grand mistakes. The plane is a plans built little pattern plane. I was talked into using the stock mounting rails instead of a normal mount. That left me no room for a fuel tank behind the engine due to the rails. You can see how far back I had to mount the tank and it is over the CG area. It didn't work!! As soon as 1/4 of the fuel was used up there wasn't enough pressure to help the engine draw and it would go dead stick. I'm just waiting to get another YS engine for this plane. They don't care where you mount the tank. I did try after market pumps but they didn't work all that well. They worked but I couldn't get them regulated correctly, again, my bad. Always CG with the tank dry!!