try National Parts Depot , Mustang and other car parts supplier .
They have a 1969 Hood Scoop emblem that would work - it says simply " 351" w/ nothing else , ie, no Powered by Ford or such wording .
This site should work :
http://www.mustangpartshouse.com/nopartsfound.php
The NPD part number is EMB-351 and it is priced at $ 23 +/- plus S/H ...each, I believe .
Other Mustang parts suppliers , or Branda , might have different 351 emblems as well . Bill