when i fitted rear seats in mine i got hold of a couple of back seats out of a Transit minibus which had the inertia reels and buckles fitted to the seats already. all i did was drill through the floor and bolt it just above the chassis with a spreader plate, one bolt thru the floor and thru the chassis bracket on the inner side of the chassis.for the outer two bolts i drilled thru the seat box and the reinforcer beams underneath the wheelarch and fitted angled spreader plates so as not to squash the outriggers.
Are you fitting genuine rear seats or are you on the look out for some, if so my next door neighbour has just converted a minibus into a van and might have some seats with integeral belts for sale, if your interested let me know and i'll ask him....