Had a similar problem with my 850 3S. I cut a much shorter power pack and put the battery in the nose behind the power pack. Face the battery wires back and route the plug up into the hatch compartment. I routed the wires for the motor through the first former into the hatch compartment.
CG was good since I was a bit tail heavy before. Unfortunately, I also haven’t been able to get this bird to fly either after one attempt, it broke the firewall on the launch a couple weeks back.