sorry just realizing I haven't replied.
I had to drill new holes in the floor. The cargo frame does not have mounts for additional passenger seats. I had to fabricate plates to attached the Ford OEM seat to. I used 3" wide 1/4" steel plate with a welded (captive) nut on the plates. I requires dropping the fuel tank, and there is another thread somewhere in this forum that covers that part. The vans beams are made up of "U" channel with a hat on each side. The hat is where the fan floor is tack welded to. I made sure to span the plate under the hat of each beam such that the plate would not be able to slide. Due to the hole pattern of the Ford OEM seat rail the holes line up right on the edge of each beam hat so measuring 50 times is key. First fabricate the plates with your holes pre punched in the plates. Then check for alignment underneath. I used an awl to mark center before drilling.
This essentially makes a clamp on the vans frame in multiple locations. I was not able to utilize all the holes on the passenger seat rail due to an upside down U channel precluding my connection. Rather than use rivnuts in the floor I decided not to drill that spot and used zero connections in that location. Instead I fabricated another steel channel of the same dimensions and hole pattern as the ford channel and installed it on the drivers side of the OEM drivers side seat rail. Connections to the frame for this new rail are the same as the others. Then I welded steel plates from the new rail to the OEM rail inside the van. This then completed the "clamp" over the Ford drivers side beam. My 2" insulated flooring conceals all the structural members.
Another trick is to leave your captive nut un-welded to the plate. Mount everything up for alignment and mark on the plate exactly where the nut should be welded to the plate. Of course you must be careful welding when the fuel tank is detached.
I make no guarantees that seat install method is flawless. Anyone else using this method accepts all risk.