I don't think it would fit, as the g-maxx is slightly longer. you may be able to modify it (stretch it out!) to fit, but it would be a PITA. I usually run my motor down all the way to one side. This keeps it out of harms way.
I run mine the same as mike does - down to the side - I would be surprised if there was any diff in the handling based on the side mount vs the top mount - In any case - I would rather suffer a teeny bit (if any) handling issues rather than replace a motor... Just my .78 cents.....
We also don't run a shift servo. The motor is placed on the opposite side of the steering servo, so the weight balance (or lack of balance) is minimal.