The seat rails are connected to a specially reinforced section of the body that is not present on cargo vans, essentially sandwiching the mount with the rails on top, the floor in the middle, and the reinforcement on the bottom. Installing that reinforcement is theoretically possible, however it is very complicated and involves removing things like the fuel tank. If you were to realistically install passenger seats, it would be quite substantially less safe than even a non-airbag equipped passenger van (a non-finished, non airbag passenger Vito is sold in some markets). It would also be illegal for road use in the US without going through substantial hoops (Actually it would be illegal even if you DID install the reinforcement).
Enforcement criminally of this is pretty lax, and afaik is only ever done if a fatality occurs because of the violation, or the vehicle is being used to commercially transport passengers for hire (think taxi/bus/livery). However you would be engaged in negligence and could be (probably would be) sued in the event of death or injury by, for example, an employee. It would also likely invalidate any liability insurance you have for damages related to it (both auto and business), so you would be on the hook for it personally. I am not a lawyer, so i am not giving you legal advice, but still keep it in mind. Were I using my potential Metris as a crew van, I would use a passenger van as the base. (Actually, I’d probably buy a Sprinter crew van, since it is larger and not substantially more expensive than a passenger Metris).
If the safety is not articulately important to you, it really doesn‘t matter where you mount the things, although I suppose I would try to take advantage of the reinforced mountings for the tie-down points. Jury-rigging something using them would probably be mildly safer than attempting to bolt in OEM seat rails without the passenger reinforcements.