Proper job. The Mamba max (MM) will happily handle the longer 8 & 9xl motors on up to 14 sub-c cells, though they need to be good cells, not cheapo ones. The V2 slipper doesnt come with a cover, but no-one has ever had problems with stones or chippings on the V1 version (same but different motor mount). Most people these days dont bother with gear covers on the emaxx tranny, and 1/8 buggy conversions have no cover for the modified gearbox. On a GorillaMaxx chassis or similar like the FLM, if you do run on very loose gravel, then it is possible to fab up a cover for the bottom of the chassis:-

Candyman's MaxxBling
Other options for speedos include the Quark 125b- but it is a little tempermental, or the warrior 9920 which is very tempermental, or the schulze 18.97k - which is great but expensive. Only other option is to wait for the Mamba monster maxx which could be another year or more at this rate, or Tekin are releasing a big truck system, which should be out fairly soon I think.
But on a fairly light truck ~10lbs or less, the MM on 12-14cells & 8 or 9XL is a good setup.