I think the University of Virginia Library may have hit the "mother lode" when it acquired Roanoke, Virginia television station WSLS' 16mm news and sports film archive and began digitizing it.
The link below will take you to September 1968 at Martinsville Speedway prior to the Old Dominion 500 NASCAR Grand National stock car race.
The late, great radio announcer Bob Montgomery , anchor of the old Universal Racing Network radio race broadcasts before his passing, interviews brothers Bobby and Donnie Allison.
This archive of WSLS film runs from the years 1951 - 1971, by month. I think RR member, TMC-Chase needs to peruse it to see if there is any Lee Petty, Julian Petty, etc. footage floating around.
The link is below and the sports anchor's script is printed below the film link:
http://search.lib.virginia.edu/catalog/uva-lib:2219652
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"Any Day is Good for Stock Car Racing"
updated by @dave-fulton: 12/05/16 04:00:58PM