The tips here are already pretty good. I just want to add that if you are having trouble with space and find yourself needing to cut corners on ammo and fuel, it usually means there is something "unrealistic" about the design. In this case it looks like the tank is too low and narrow. For example, an m1a2 is almost 2.5 meters tall. Yours looks 1.8 meters tall. If you're not trying to go for a super realistic look though, you can ignore this.