How to get the face of a part?

Im trying to make a grid placement system and it all works but I havent figured out how to get a model to be placed on the face of another model. Theres a Hitbox in every model and id like it when a player presses the build button for the new model to be put in the direction of the other model's hitbox face whenever the cursor is on said face. Ive tried to work with LookVector but im not sure how to apply that to the face of a part.