the gasket goes between the manifold and your prochamber on the passenger side. I would either re-use your original gasket or stop by ford and get a new one, ive heard the gasket mac provides isnt that great of quality and will eventually blow out
i think they may have changed the gaskets at the same time they changed the coating on the prochambers, cuz mine was real thick, and at least LOOKED better than the ford piece, it may just be me but it looks like mac has addressed a lot of quality issues lately, anyway got mine mid-summer and no leaks yet, id say use the mac piece, worst case u gotta take it off and put on a new one no big deal