I went to west coast cougar site regarding this question, and here is what I found:
Your Name * :
Steve
Reason for contacting us * :
Can't find what I am looking forOrder number (if applicable) : N/AComment / Message * : Part 26043, FILLER BOARDS - TRUNK DROP OFF - PAIR - REPRO. Reproduction Trunk Filler Boards for the 1971, 1972 and 1973 Mercury Cougar. Will these fit a 71-73 Mustang? Thank you, Steve This is the response I got:not a direct fit, but you could trim with a box knife to work.
Darrell Niemeyer
Technical Support Dept.
Direct line: (503) 428-5836