If you are looking to swap in a 79-80 skid into a 78 Invader, I believe they will just bolt in with no modifications required. If you are looking to swap in a Vari-Ride from an 81 (or any of the LTDs) it will require modifications. At that point, unless you are a purist, I would suggest swapping in a ski from a 90's Polaris. They ride better, and from what I hear they are an easier swap.