As kind of an expert on this gun, I would suggest going a different route, replace the high speed motor with a high torque and stick with your current spring. I suggest either an SHS or ZCI motor, a Lonex A2 Orange is also good but pricey and the other two can be found much cheaper and do the same job. As for shimming, these gearboxes are a little more difficult to shim because of how they are designed, but they're not impossible.
Start with your motor in position, set your bevel gear in place, and check for clearance issues between the bevel gear and the motor's pinion gear. You should also check motor height adjustment at this point, too. Add shims as necessary to the bevel gear between it and the gear box. Once you have this adjustment aligned, remove your motor, close the gearbox shell with the bevel gear in place and check for gaps between the bevel and the other half of the shell. Add shims as necessary, but do not over shim, leave a small amount of play, about one to two sheets of notebook paper.
Once you are happy with the bevel gear, add the spur gear and shim and adjust until it is aligned, including closing the gearbox and checking for clearance and tolerances. Each time you close the gearbox, check the gears to see if they spin freely and if they shift side to side. Repeat the process for the sector gear.
Mind you, you do not need nor do you want any other components in the gearbox while shimming, they will only interfere and make things more difficult. Also of note, use SuperLube instead of Lithium or silicone oil or grease, it is much better and lasts much longer. You can generally find it at ACE hardware and sometime Walmart, you can order it from Walmart online. You want the gray tube that reads Synthetic Lubricant under the brand name. Use it on the gears and the piston slide, but do not use it inside the cylinder, use silicone grease or oil in the cylinder.
I also suggest correcting AOE with sorbothane and piston tooth to sector gear alignment modifications. These will greatly improve your gearbox's performance.
Hope this helps, and if you have any further questions, feel free to ask.