Larry,
The Digitec retracts come with adapters to fit the 1/2" strut opening of the 600 series, basically they are steel bushings with rods that fit the drilled strut, I believe they are $150 USD
The other option is to by the "Small" set of trailing link struts from Dreamworks that are made to fit a 1/2 retract direct, I have both and they are equal in quality however I do prefer the direct method.
If you look back through this thread you will see that I cut open the gear mounting area and made 1/4" plywood ribs and rails to mount the retracts, the off the shelf kits have crap wood glued directly to the fiberglass, you may get away with this if you can land like it like a feather; however the way I put them down the stock set up would not last past the first landing
