The 11" drum brakes and 8.75 rearend are not a lock for it being a Road Runner; I had an original '70 Satellite wagon that had them.
That car was ordered like a wagon version of a 383 RR .
You might look at the quarter panels aft of the scoop area: Road Runner would have 4 holes on each panel for its name plates.
It would also have brackets for dual mufflers, though that's no guarantee it's a RR.
The partial VIN under the trunk weatherstrip will be towards the rear of the left quarter panel, but what you need to find is a full VIN stamp starting with RM.