Would like to get some more oomph from my 318

Yes the final ratio in 3rd will not change with the low gear planetary in the transmission. It only lowers 1st and 2nd gear giving you a little bit harder launch. That and with the 8.25 axle with 3.55:1 gears will change the whole feel of the car. Do the gears and axle first and you may be happy enough with the results and leave the rest of the motor alone. Till the speed bug sets in again and you want to go faster in the future of course!