Ben:

I have the plans for the BT Hellcat. This is how I would go about installing retracts. I do not have the plans in front of me at the moment so I cannot remember the rib numbers that the retracts would fit between. Anyway, take those ribs and use them to make templates. Cut the ribs out of 1/8 aircraft ply (5 ply). Cut out four plywood ribs for each rib. Sandwich the balsa rib between two of the plywood ribs and glue them with 30 minute epoxy. You should now have two ribs per wing half that are doubled with plywood. Make a 1/4 plywood plate that will fit between the two ribs and cut out the center section of the plate so you can mount your retract to it. You will also need to re-enforce the main spar area around the retracts because you will need to cut a "U" shape section out of it in order for the strut to clear as the wheel is retracting. You can do this by making two 1/8 plywood plates that fits in between the top and bottom of the main spars and is glued to the front and rear of the spars. I would extend these plates from the center of the wing to one or two bays past the retracts.