Another ADO16 owner here. According to the UK dealer brochure from back in the day rear seatbelts were a factory option on these meaning that there should be threaded mounting holes for them already in there somewhere. Might be BS cos I've never managed to find them on my Wolseley 1300 but have a look anyway and if you find them please let us all know where they are.
The cars in question are 19 and 26 years old. A Toyota and a Nissan respectively. Both are second hand imports. Would the appropriate franchise dealer be able to access the required information?
I found the factory jack with the sticky uppy bit that fitted the orifice to be a fool of a thing. Never felt entirely safe using it. A trolley jack under the subframe is far superior.