Hi Tom
I also installed one retract yesterday. I cut my own Hardwood block from maple which are maybe 3/32 less high than the included blocks. I actually bit into the wood to see which one is harder )
I am also using 3 1/2 inch tires and had to extend the wheel well to accommodate the larger wheel. I am also thinking of creating a balsa sheet ring to insert into the wheel opening for additional strengh. Will try this during the weekend probably. Anyway, the retract sits low enough in the wing that only the topside of the struts are a bit above the wing surface. This also provides me the opportunity to slightly tilt the retracts forward with shims if I have a nose over problem.
Let me know how your maiden went.

Pietro