I posted a reply to a similar question in the Van Body sub-forum----reposting it here in case its helpful:
Recently mounted a full size Snap On tool box to my E250 floor. After routing a recess in the 3/4" plywood floor to accommodate the box's footprint 7/16" hardened threaded rod was used to capture a 1.5" equal length leg L aluminum angle, 3/16" thick. As seen in the photo below these pieces fit snugly inside two floor ribs which I believe are quite strong points in that area.
Were this for seat mounting I'd do much the same but maybe use 2" equal length leg L angle no less than 1/4" thick.
While the steel plate or large washers are good ideas I believe my method ties into stronger parts of the floor, not just a relatively small area such as 4" x 4" etc.
Obviously anything other than bolts and washers only through the single layer of the floor is better.
As to vehicle inspections I have no idea how this would affect that process.