I've done this conversion. First, for whatever reason, the outer bench seat bolt holes are too far outward...by about an inch and a half or so. They CAN still be used, but it makes for a VERY tight fit for the seat belt retractor.
Second, the the inner brackets should be welded to the top of the floor, not under, with the humped part toward the front. No drilling through the floor is necessary for the bracket installation.