Sarks,
I do not have the Sierra retracts (Century) however didn't have to cut much of the balsa away except for a larger diameter tire. The only thing I needed to do was cut out some of the spar for the piston to sit nicely. I went the opposite way in reinforcing and cut away a section on the top of the wing. It was easier to fiberglass everything from up top (for me). Others might have better recommendations however.
For my scenario I was able to test the strength after vibrating off a wheel and landing gear down. I bent the gear and the wing held together solid.
I'm assuming every install is going to be different with alternate methods. For me, my dremel tool is my best friend.
Jim-