I put them into my '73 recently. I went down the rabbit hole of trying to find hidden bolts mounts etc, which apparently are on some years and some European-issue models, supposedly. In any event I did not have factory mounting points, but they were easy enough to install. I bought the kit from
www.quickfitsbs.com in the UK (there are other providers, they were easy to deal with and knew the model.) While you typically want to hide the retractor under the parcel shelf, there is not enough room on an E3 (rear wheel well is too close to the shelf), so they are mounted on top, but you can pick any color vinyl covering you want for the decorative box used to hide them, and it looks pretty good. To mount there is a bolt through the parcel shelf, and under a plate to spread out the pressure, then you use the existing hip mounting points for the factory lap belt and you're good to go. Lets me put my kids (including car seat for the younger one) in there, otherwise I'd never get the time to drive it. The attached picture shows the box hiding the retractor, its actually not that noticeable and belts have been comfortable to use.
