I recently secured a full size Snap On tool box to the floor of my E250. I first routed a pocket slightly larger than the box's base in the 3/4" plywood floor before bolting it place. This photo shows how using a piece of 1.5" x 1.5" x 3/16" aluminum L to span and capture two strong points of the van's floor system:
Were I doing this with seats I'd use maybe a 2" equal length leg piece, perhaps no thinner than 1/4" to similarly capture the stronger parts of the floor.
BTW the threaded rod shown is 7/16" diameter hardened, B7 grade (although I have no idea what that spec means.)
Hope this helps.