I want to level the stance of my G55. If I replace the front springs with G63 springs or 30 mm spring spacers would I have to replace the shocks as well?
The front is lower than the back and I just want to make them level.

 

Are you taking it offroad, or just street driving? On road, you should probably be fine. Offroad you would want to check the suspension travel and be mindful of whether the shocks top out or not. Keep in mind that shocks are not desinged to limit suspension travel, and you can damage them from repeated over extension. Given your question I assume you have already contemplated that though.

 

The G63 front springs are 1" taller than the G55 front springs when they are off the G. When installed, somewhere in the range of 0-1" taller. So a MAX 30mm spacer will help level out the rake/stance. FYI, the rear G63 and G55 springs are exactly the same.

As for the shocks, the G55 shocks have internal bumpstops that make them much harsher than normal G500 shocks. I believe the lengths are all the same length so you should be ok there, but if you're looking to increase comfort you should plan to swap out the G55 OE shocks. (I can measure them for you to compare if needed as they're in my garage) If you're planning on changing out the front springs or adding front spring spacers, you've already done 95% of the work to replace the front shocks. The rear shocks are also pretty quick to replace and should not add much labor. Also I would recommend switching to G500 springs if you're going for a swap. I swapped to G63 springs and Fox shocks and the G63 springs are still too stiff to be comfortable. You have to run the Fox shocks at 4/8 clicks from soft to pair well with the G63 spring rates and in doing that, the G rides pretty rough (although MUCH better than the OE G55 suspension). Your area roads may change that formula. I believe a good set of shocks have a much larger effect on the G's handling than the springs. The springs are the primary driver in overall comfort - feeling all the bumps on the road.

Any single tube replacements will all be bolt on and be the easiest - Koni Raid, Bilstein B46, G500 shocks, etc.
Any Adjustable setup will be more involved - Fox will need to cut the inner fenders, kings will require relocating a secondary top mount, ohlins I believe bolt up but are the most expensive - but are supposed to ride the best.