i just got a xxx to be a little bro to my xxx-s, and i am going to run it on carpet, but i have the slightest idea. I'm thinking the same dimensions as my x-s, stiff springs all around, foams, lower it, stuff like that.
I am not sure what you will find for foams for the XXX buggy. We have had great success running Proline Road Hawgs or Rages on the buggies and trucks. Just make sure to buy some traction compound and use that on the tires. You will also want to stiffen the suspension and use alittle heavier oil. You don't need to go too crazy, just start going up 1 spring rate at a time and see how she goes.
You may also want to lean the rear shocks in, but it really depends on your driving style.
Since you got it in truck form now, you should look at making the rear end less stiff if you are using foams. The foam tires don't provide enough rotational mass to cause the rear to come down and hence cause the truck to nose up. She will start jumping nose down pretty hard. that is what we are experiencing at our track. Got alot of racers so ticked that they went to rubber tires.