jatmon

i am working on an rx8 for a 24 hours of lemons car, for those who have raced these, looking for a recommendation on fuel cell location. i do have the option of using the oem tank, but would like to increase the fuel capacity. will mounting a fuel cell behind the rear axle in the spare tire area be bad for handling? i hate to put all of that weight back there, what with the weight of the fuel, cell, and structure to make it safe. i do have the option of going in the cockpit just above where the factory tank would be, but it will be higher up. which option will have the least negative affect on handling?