Here's what I decided to do to install the Sierra Giant scale retracts. The problem was the rails provided didn't leave enough space between them to set the retracts in and if I made them narrower it wouldn't leave much meat for the screws that hold the retract down.

I opened up the wing and trimmed the sides of the retract bay where the bottom slots into the sides. See picture 1.

I marked the back of the mounting blocks supplied and trimmed out the outline. This allows the full width of the block to mount the retract.

Then I cut some 1/8th aircraft plywood (5 ply) to epoxy on the back side to help strengthen it. I still have to sand out the spar for the air cylinder and cut the bigger hole for the 4" wheels I'll be using. I plan on using the carbon fiber idea that HalH showed in an earlier post. I'll also beef up the original structure with some more epoxy.