-wheels are indeed too large
-round extenders are indeed better
-you need to use the large black batteries for that. two should be enough, or one black battery plus a nifty. in stock avoid the pink batteries, and nifties are limited in their effective use AKA lightweights and some low power middleweights, and the occasional "i need a bit more power" moment.
-high power Ztek drive is needed, instead of normal ztek drive.
you got the no empty space rule well.