68 and up is a lot simpler than 67 and earlier.
I don't know if this quarter wall by the window winder is/was strong enough for a seat belt anchor. When factory did similar 3 point in 74 and later models they routed the belt through the panel and put the retracter inside and bolted down at the rocker. If my memory serves, they had to limit how far the quarter window would roll down also.
What you want to do might look good on paper but... will the belt pull down on or slip off your shoulder? Will it be all kind of in the way going in and out of back seat? It is sort of like convertibles but those have high back seats and belt routes through a loop mounted high on the seat. That belt still gets in the way of going in and out back seat but at least it doesn't pull down on or slip off shoulder.
A test to see will it work, will you like it... Pull interior trim as needed to put small c clamp or some sort of temporary tether there. A length belt, rope, whatever. I would route the belt down to a brick in the floor so to see how its in the path. Let us know what you decide.
