Nutserts into a 5/16" thick structural member are a completely different animal than ones going into sheetmetal.
If you look at the test strength from Rivnut (in my experience the best nutserts), the sheetmetal tests are done on a panel that is supported on both sides 1/2" from the centerline of the hole. It is not until they get to 0.125" plate that they test the strength against just the mounting surface.
A fully compressed nutsert will have a loading surface that is an annular ring that is a percentage of the diameter of the bolt. It usually ends up being slightly smaller than a standard diameter washer (as opposed to a fender washer). I have not seen your specific setup, so I cannot comment on its structural stability.
As a generic recommendation to someone looking to move a mount, I would say to look at how the stock mount is supported and at least duplicate that. On my truck, the underside of cab where the seat bolts go in is a much thicker section of metal around the bolt. This could be replicated in another location by putting in a backing plate. The purpose of the backing plate is to spread the load out across a larger surface area. The strength of the metal is correctly measured in lb/in2, so increasing the area where the load is applied will reduce the likelihood of structural failure under load.
Last edited by Fibonachu; 08/11/2021 6:45 PM. Reason: to add more detail on backing plates
