I wouldn't recommend the 550, unless your running 2 pole. The 540 motors out there are more than enough for this truck, and the extra weight of the 550 is not needed or good for the setup in my opinion. I personally run a hobbywing 4700KV XERUN motor with a 15t pinion. It flys! This truck is already nose heavy, and generally biased to the left side in weight with heavy servos. If your truck is nose diving, take out the antisquat and the battery back mod is highly recommended as well to shift that sucker back 10mm.
If you invert the pistons it will slow down rebound some to and has a more stable effect.
Try running 6 hole 1.5 all way around inverted. This will increase the pack rate and slow down the suspension rebound travel which will allow you to bring some antisquat back into the mix. These things should knock all that nose diving right out.