Bigbird I did the samething I also am using Sierra Retracts. One thing I did was to round the top inside of the gear blocks to match the curve of the retract to give the most possible contact area. After cutting the needed amount from the spar to give the clearance for the retract I felt that too much had been removed so I took 1/4" ply and added a sandwhich on each side of the spar for the added strength. Then as you I made new 1/8" ply ribs for the gear blocks but I also used triangle stock at each ends of the ply ribs to make sure that the whole landing gear blocks are very secure. then cut out for 4" tires and removed all needed balsa ribs in the wheel area but added balsa ribs on the sides of the wheel well to give strength. I have just finished glassing the wing should be in primer this weekend good luck.

Randy