Stop in for a cup of coffee

So.. i need to make seat belt mounts for my car as it didn't come with any... i see guys using just a bolt and big washers.. but i was thinking of doing like a 4x4" plate with holes in it and plug welding it to the underside of the floor.. am i overthinking? also.. what bolts do seat belts use 7/16-20? Also.. need to find some gray seatbelts :)