Block group from circa 1970 reaching number one (8)

'number one' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe you can see an association between them that I don't see?

'block group from circa 1970' is the wordplay.
I cannot really see how this works, but
'block' could be 'stop' ('stop' can be a synonym of 'block') and 'stop' is located in the answer.
'group' could be 'o' (O is an example) and 'o' is present in the answer.
The remaining letters 'dor' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

'reaching' acts as a link.

Can you help me to learn more?