This has been discussed a few times previously so perhaps do a search on "rear seat belts" or similar.
Children and seat belts is a critical combination. If I were in your position I would get the advice of one of those licensed seat belt installation places, a reputable one. They know the laws, as well as the practical and sensible options given the age of the car.
Having said that others on here have done their own thing.
I've posted the BMC engineering drawing for the rear parcel shelf elsewhere showing the welded bracing added for strength as seat belt laws developed. Let me know if you can't find it.

Edit: Here -
viewtopic.php?f=27&t=93990&p=1010599