Thanks,
The receivers mount directly to the new seats. trouble I am having is the other side of the lap belt part. Just attaching it to two little M6 bolts does not seem to be very substantial. (see also this thread
http://forums.pelicanparts.com/showthread.php?threadid=156048&highlight=seatbelt) I spent a lot a time already searching the archives, but have not come up with the definitive answer.
I was thinking about either welding plates to the sills, or otherwise bolting them directly to the floor.