Assuming you are stuck with the brand new gel batteries you mentioned in the first post, you can add a 7th gel battery w/ Rodney’s hack or, if Dave (@Inwo) is willing to build another one, you could go with an auxiliary LiFePo4 pack like I did. They both achieve the same end, the wiring is different though. I can hit 40-41 mph with a 4s pack in an '02 longback w/ 6 lead batteries for the main pack. You should have a T4 controller, which can handle more volts and higher amps than the T2 in mine, so you could probably get away with a 5s or maybe even a 6s (Dave or someone who knows more about that controller could help you there).