Also because you have the fuel tank in the trunk too. You must have a firewall in the rear section. As for having the fuel pump mounted outside the car. Well be sure it is not in plain view under the bumper facing out the rear. Yeah it will stay cool, but any mild rear end hit and your going to spill out fuel all over after the pump takes a hit. On a street car mounted outside the trunk, mount it on the inside of the frame on the side of where the stock tank would go. This way it is protected a bit from most rear end mishaps.
In my '70 GS I ran two pieces of squared aluminum that were the length of the fuelcell (running front to back of the trunk). I ran the bolts from the fuelcell through the aluminum mounts and into the trunk floor. It's a simple mount and I polished it with the aluminum fuelcell.
