I'd say the 2650 if you NEVER plan on running 6 lipo. Either way it will be amazingly fast. The 2650 will be perfect for 4s and maybe 5s in a 8th scale touring car. Look at the other conversions before you make your decsion. There are a few serpent ones running around out there. One is called "the 1/8th scale onroad think tank".
So... is it the one i need for the gear to fit into those 5mm shaft? will it fit the stock Kyosho pinion gears?
oh god, i really dont know nothing about conversion, please help