See the link below; you should have the upper and center tunnel anchor point already in place, but you probably have to drill through the floor for the outer lower anchor point (i.e make it from scratch)
I did the belts on my 63 W111 and ended up using only the upper anchor point.
I put the roller on the floor right at the B-pillar (new hole)
I fastend the (quite long) latch on the underside of the seat and looped it up on the side of the chair. when the latch moves with the chair it is always in a good position, rather than to much or to little depending on the adjustment of the chair.
I fastend the lower anchor point on the lower backrest adjustment frame on the outer side of the front seat, yet again, in order to enable it to move with the seat.
feeling worried that fasten it on the chair might be a bad idea in case it comes of in a crash? Dont be! the seats were designed to catch the rear seat passengers so they wouldn't fly out on the front seat passenger (who would safely land on the very soft padded dashboard. (that's why the seats are so insanely heavy...)
Let me know how it goes!
Animated publication
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