The Dynamite 086 pipe is quite short. It should fit in the MBX6 no problems compared to some others. You may have to enlargen the stinger cut out in your body, as the 086 stinger will sit a little further back than most longer pipes.
Don't worry about the bottom end with the 086 on the MG66 man. I run a small tight track and a larger more sweeping track here, and on the small track with jump sections just out of the corners etc I have more grunt than I know what to do with. In fact I struggle to keep the front wheels down on a grippy track. That's with an Ascendency 4 shoe clutch running 4 x gold springs, a 7mm restrictor and .4mm shim stack on 30%. That gives me power to burn and 10 min tanks if the ambient conditions are right.
You are going to like that MG when it's done a few more tanks mate