Rear seats. New buckles attached to same points as Toyota's centre lap belt, which is retained.
Seat belts
I have fitted four lap and diagonal inertia reel belts in the back of my '91 Royal Lounge. What you need costs about £150 from a good car parts shop (nearer £200 from Halfords). Britax/IMMI don't make any that are long enough. Buy Securon: four SEC 150/15 auto front belts at £27 each and two SEC 210 static rear belts at £13 each. Both types have the same buckle. The buckle side of the static belt is on a long strap which is needed for the two seats on the back row and as a bonus the other side of the static belt can be fitted on the middle row for use facing backwards. The 150/15 has the buckle on rigid strip 15cm long which fits well on the middle row - I bent mine outwards near the top about 10 degrees for comfort.
Rear seats. New buckles attached to same points as Toyota's centre lap belt, which is retained.
Middle row. Replace Toyota lap belts with Securon ones, using rigid buckle stalks which will also accept the shoulder belts when facing forwards. Door pillar fixing, note proximity of rear-opening-window latch
Fixing the inertia reels each side for the middle row.