I use a 3/16 steel strap and 2" washers when I relocate seat mounts on sheetmetal.
That won't work for panel truck seats; depending on where you locate the seat, due to baffles inside the pedestal the inside is inaccessible with one's hands to install nuts/washers. I know because I tried it first before I resorted to nutserts.
A wreck would not pull my seat mounts out because my seatbelts are mounted to the floor behind the seat, not the seat itself. And simple math will tell you that the cumulative strength of eight 3/16" nutserts is greater than two 1/2" seat belt bolts.
Nutserts into a 5/16" thick structural member are a completely different animal than ones going into sheetmetal.
The original bolts are in sheet metal.
TCI Model A frames are only 1/8" wall, nowhere near 5/16. TCI was originally criticized for using nutserts too, but time has proven them secure.
This is another case of guys warning against something they have never actually done themselves. I've done it, it's worked flawlessly for three years with two-up in the seat and a bunch of horsepower shoving us against the back. Yeah, were both lightweights; maybe those with heavy loads might not want to try it ;-)