put mas file in rFactor 2\Installed\Vehicles\Porsche_911_GT3_R_2018\x.xx folder where x.xx is latest version's folder you have in there. when S397 updates the car just move the mas file in new version's folder
put mas file in rFactor 2\Installed\Vehicles\Porsche_911_GT3_R_2018\x.xx folder where x.xx is latest version's folder you have in there. when S397 updates the car just move the mas file in new version's folder