Here are the seat belt anchor brackets, with nutplates installed, attached to the underfloor ribs with the prescribed blind rivets:
I made a pair of little covers out of 0.016" alclad, to cover some of the big hole in the seat pan. One end has a little slot so they can be removed easily along with the floors. I think they will look classy when they're painted.
Sometime next year, when I can't find where I put these little covers, somebody please remind me that they're in the box with the harnesses.