the fc frame and tank don't have the mounting tab on the front and the fc blocks off the front air filter. the tank has two petcocks. if you're considering putting on the fe tank with the long right side, it will fit without the front mounting bolt, but i'm not sure the frame crosspiece above the swingarm has the mounting tab for the bottom (the doughnut steve mentioned). also you will have to find a vacuum port in front of the carb to power the fuel pump. and this tank only holds 0.1 gallons more than the fc. hardly worth the effort to change. plus with the fe tank, you lose easy access to the carb.
now if you were referring to the acerbis 5 gallon, i believe it just mounts like the fc, but don't know for sure.