OK - there will be more recently informed people than me that can comment
I am taking my 66 over shortly, has had the lap belts in the front replaced with retractables (relates to your question about fitting), from what I understand you will need an engineers cert. for the change (I am about 2 weeks away from inspection)
In terms of no belts in rear - I used to have a 65 (many years ago), had front lap and no rear - was fine for rego (as it was standard), that was over 10 years ago though
cheers
greg