i made this drawing up the other day. no, my brackets aren't actually these colors, it's just to show the layers of metal.
all of the bolt holes are roughly where i have mine at. i recommend using all six mounting points on the seats for strength. the black pieces are the origional seat rails. the blues are the cross sections to make the frame. and, i also put the red metal sections on because even with the seats back the whole way, they seemed to sit too far forward for my comfort. would be nice if i made them so the seat went back a little further yet. you'll need some type of spacers or washers because the bolts for the rear cross rail will interfere with the mounting of the seats. so, that's how i mounted them back farther on the rails, otherwise you could do without the red metal sections.
i bought some square tubular welding metal for the cross pieces that bolt onto the seat rails(blue pieces). i think it was 1/2", just big enough to fit the size bolts that i used to bolt to them to the rails. i also bought some thin flat metal (red pieces). it might have been 1/8" thick X 1 1/2" wide. strong enough to be sturdy. i can't remember what size all the bolts were, sorry. might have been 3/8" for the rails and 1/2" for the seats.
take the seats out and unbolt the rails from the bottom of the seats. i bolted the rails back onto the floorbards and measured the length needed to cut the metal cross rails. looking at my drawing, i think the rest is self explanitory. measure twice, cut once. same with the holes that you drill. you do have an angle grinder right? also, i think i ended up cutting a couple of inches off the front of the seat rails because they stuck out a little bit.
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