Yes the GP Mustang wing uses the same ribs as the Ultra Sport 40 wing, well almost except the few of the root ribs to give the wing the mustang wing look, and the two kits builds just about the same both the US 40 is fast and nimble and the Mustang is fast and nimble so they are really close its just that you have a Ultra Sport on one hand or you have a Mustang on the other hand both are great flying planes just follow the build to the letter of the book and you wont go wrong. In my preference DO NOT USE SPRAY FOAM IN THE WHEEL WELLS if you wish to go with retracts there is no need honestly for the spray foam. If you build the retracts and as you are cutting away the wood sheeting and ribs for the retracts yeah you will end up with all that area in and around the ribs, what you do then is to get you a cereal box and make the wheel well openings with the cereal box. The cereal box is card board and sandable and covering sticks to the card board cereal box and CA glue sticks to the cereal card board box. If you need pictures of what im referencing to let me know and ill be happy to upload of what im talking about. Good luck on your Mustang and post pictures as you go and the final product. There is also a P-51 Mustang brotherhood thread.