well grind off the humps on the chassis and then u can use two long center revo shafts witch are cheap and work very well, and for the motor i think u want a 8xl the 8l is smaller and it will heat up faster
if you are going to use the slider centre shafts (which is a good idea) then its not that critical where you drill the holes as long as you can mount the tranny... the sliders alow you some flexibility as to where the tranny sits