That rig is far too heavy for the mamba motors. Get a larger (Fiegao XL or Neu) motor and you can use the MM on 4s with external bec. You will likely burn up the MM motor in a short time.
The 8xl will give you longer run times and tax the MM less but be slower. Although you could gear up a little to compensate fo the lower KV if you wanted.
8xl is very fast on 4s. On 5s it's pretty stupid, lol. I'd think it would do well in the savage. I don't think i'd try a 7l, I'd think it'd run way too hot. A 9xl might even be a better choice, since the savage X is going to be quite heavy.
J.
*edit*
I see you want to run 3s. nix what I said up there. You could try a 7xl on 3s, but I think you're going to want more voltage before it's over and done with.