putting '99 60/40 bench in my '68??

[superfly68]

the backing plates go under the truck. imagine for a minute that a chain is wrapped around your seat through the windshield (for visualization purposes it's not there) and attached to a towtruck. now imagine that towtruck taking off and ripping your seat out! well, if you have backing plates installed it has to pull those through the floor in order to get the seat out. SO, your body will most likely stay in the seat (if you're wearing a seatbelt) and the seat in the truck if you got in a bad accident with the backing plates installed. otherwise your seat might come completely out of the mounts. hope that clears it up for you.

[matapalo]

The backing plates that were on the original stock bench seats in my truck were 1/8" x 1 1/2" x 3". I upgraded to 1/4" x 3" x 4" steel plates in most cases. No need to weld as the mounting bolts will hold the brackets in place.