"The 1988 and newer models have some of the original seat upholstery heat laminated to the foam making it very difficult to remove the old upholstery. Our seat kits for these models do not require the removal of this laminated upholstery. Our sets will go right over the original and hog ring where the original seat cover is attached."
To me...that's not seat 'upholstery'.
That's a seat 'cover'. ...and it's not OEM fabric. Relatively close, but not the exact stuff.
Most of the seat cushion upholstery on 400's are attached via long plastic clips that run around the outer perimeter of the original skins & snap on to the metal frame.
Very few areas of these seats are held on with hog rings. One or two, maybe 3 spots, depending on whatever year, style & either bench or buckets.
There are no provisions on these seat cushion frame pans for the use of hog rings other than the factory requirements.
Velcro or glue is used inside both the cushions & backrests on the foam faces/seat skin backs to hold the skins down to the foam, giving the seat surface definition. Holds it down in all the right spots in order to get that form along with preventing the skin from moving around on the foam face.
Zippers for closing the backrests on both buckets & benches.
I wouldn't consider these covers real 'replacement' covers. There's no way that the fit will duplicate the factory originals without that interior velcro (or glue) & the plastic perimeter clips. Going over top of the existing cover, where would the aftermarket covers attach on the faces? Where would the cushion covers attach if the factory seat cushion skins with their clips are already in the way?
$400.00? Just for the covers? Then your time? The frustration? ...and it'll never look just right either.
If that bench seat was to show up at my door, drop it off in the AM, be ready by noon the next day. $200-$250 more. Your choice of colors in a decently tough long-lasting fabric. Along with any other issues the seat may have, addressed at the same time. Lube & check that the tracks are working right. Whatever foam repairs required.
My rant for the day! Lol!