I know the current fad is to fit forward facing seats but old fashioned bench seats are simpler and take up a lot less room. Easy enough to fit lap belts for them too - as mentioned above just bolt them in with spreader plates below. My MOT station, who deal in nothing but LR also say that they are NOT part of the MOT so the degree of strength you build in is entirely up to you. I used spreaders about 100mm square below each bolt. Legally I think that the requirement for forward facing seats only affects vehicles built after 2006 (?). ( Sorry, Piper 5 posted while I was writing.! )
The window kits advertised are fairly simple to fit - but the old line about measuring twice, cutting once is very apt ! Measure and mark where you want the window, drill a hole large enough for your jig-saw, whizz round..... the rib inside will be a pain but its only held by a couple of spot-welds and some rubber glue stuff so comes out without too much effort. Offer up the window, drill appropriate holes, remove window, appply bathroon silicone, re-offer window and pop rivet into place. (I used tiny counter-sunk screws & nuts instead). On the "delux" kits with a rubber finishing strip the inner bit pushes into place with a thin ( blunt) chisel or similar & some washing up liquid as lubricant. The professionals use a triangular wire guide.
Is it as strong afterwards ? Well, my 90 rolled a full 360 including bouncing on the roof at maybe 40 mph. The roof and the rear tub creased up a bit in one corner but the side panels with the windows were rock solid.
Good luck !
