Wilga ... with the 65FS I am sure you have more than enough juice on tap. There is no need to worry with a bit of weight on the tail I'm sure ... I tried to get my battery pack as far back but the fuse structure there is too fragile to hold the battery safely. I ended up attaching it to the ele/rud servo tray, the end towards the tail. I just secured it with some zip ties and made a little tray to let it rest upon. I did stall my plane on take off (bad engine tuning) and the plane came down. The battery pack moved and damaged the fuse a bit.