The bolt hole might have a "sticker" over it, or it may not be drilled at all (probably what you will find).
The cross-brace under the cargo area will probably have the nuts for the seat belt bolts...or are the nuts on the bottom side of the cargo floor??? (I can't remember for sure), but either way........the holes for the bolts will probably need to be drilled through the top side of the cargo bed area.