1st Welcome to the board! unfortunately Hardcore has had many problems with the Mutant Maxx and all they do is deny that there is any problem. The GS Storm center diff spur gear is too small and you cant get a clutchbell small enough to make it work. If the kit is new return it before you get into the real headaches. Get the JT hybrid chassis or a Monster Pirate and Proline Bulkheads for the Tmaxx suspension and build a hybrid that will work. If the kit is not new and you got it for a really good price then what you need is get the center from an Ofna Monster Pirate or Dominator (around $30. on ebay), the spurs on those centers have 65 and 62 teeth respectively this will allow you to use the clutchbells from 12t on up for however much speed you want, also you can get a 2 speed for the MP/Dom center if you want. These will not drop right in, you will need to re-drill the chassis to make it work. just for an example of what I am talking about the Storm center spur is 51t (I believe) and the smallest bell is 12t that will equal 49.43 mph with stock Maxx tires, now the MP spur 65t + bell 12t = 38.78 mph these are calculated at 35,000 rpm's.
Please post how it works out.