Hi John,
First let me say that it doesn't matter what she looks like now, it's what she looks like when she's finished that folk look at.
I think you are safe going with the Z number that you have in mind.
With regard to the history if you don't have the old time log book which shows all previous owners then I can only suggest that you contact the previous owner and try to work back from him one step at a time.
If you need it I have a copy of the origional workshop manual which I have put on CD so if you would like one then let me know your address and I will send you one, will only cost you a pint when we meet.
Regards,
John.