If you take a look at the photo below you'll notice that the axle of the Extended Trucks are much longer than the standard MBoards Trucks. The longer axle allows for larger All Terrain wheels and the ability to slide standard wheels farther down the axle so larger motors can fit side by side.
If you are planning on using standard longboard wheels, you'll notice that the axle is too long to simply tighten the axle nut to secure the wheel. If we want the option to add large All Terrain wheels we'll have to work around this a tiny bit. Luckily it's super simple.
Using a spacer speed kit, or a stack of washers, fill the space between the hanger and the non-threaded section of the axle. This will push your wheel down the axle and allow you to properly secure your wheels to your trucks.