Tom:
Here are some quick pics. Retracts just delivered so I laid them into the wing retract opening area just to get a quick idea how they would do. Looks great to me. I will need to grind off the inside edge of the raised wood base blocks (not the flat wood base plate. This will allow the retracts to sit against the base plate for additional clearance to move gear forward-which will then move the axle dead center of original wheel location. Yes I will need to build a standoff block onto the original base and blocks due to the top mount design of the Sierra, this should be easy to do. One could drill the blocks to use original blind nuts in base to hold the blocks rather than glue. I dont see any need to modify surface of wing, it should all fit just fine. I have just sold my house and moved into an apartment so it will be a bit before I get tools out to finish it.
As you can see they are typical Sierra gear, very robust and well designed. I expect them to look great and hold up well.
PS: I personally will just epoxy blocks in place, I feel this will be very strong and I had rather the block glue joint fail on a bad landing rather than tear out the base plate and severely damaging the wing.