Vintage Thunderbird Connection (a parts supplier) in Phoenix, Arizona sells all the proper engine decals for the 66. Included in their printed catalog are diagrams showing their correct placement in the engine bay (according to the Vintage Thunderbird Club International). The diagrams aren't on their website however (www.thunderbirdconn.com) so you'll have to call and order their catalog ($3.50) at (602) 997-9285. It's worth the cost!
