For suction, you want the centerline of the fuel tank to be as close to the centerline of the carburetor as possible, or very slightly lower. If you put it anywhere else, you may need to use a pump, especially if the tank's much farther away than behind the firewall.

Please realize that you will have to use large-diameter fuel tubing and likely have to re-plumb the tank to use larger tubing. Regular-size tubing has too much fuel-line drag, and will cause the engine to run lean from the restricted fuel flow.