What is dia, length and correct finish foe tie down plates fasteners.? 67 Metuchen. I've seen the previous posts gibing a variety of finish responses.
The hardware used for tie down plates was almost anything that fit. The bolts were 7/16-18 x 2 1/2 long, hex, probably the same that were used for engine to transmission mounting. The nuts were generic, a washer may or may not have been used. It was a haphazard installation. The finish varied from natural to zinc to phosphate to early American rust. Remember, the tie downs were supposed to be returned to the assembly plant for re-use. A lot weren't.
If you choose to add tie downs, I would recommend the bolts be clean, and either natural or phosphate. Nuts "could" be clear zinc plated as that finish was common on many 7/16 nuts. Phosphated nuts were also used. Make sure they are all the same. Don't upset the judge inspecting the undercarriage.
Look in the library, "Mustang Hardware Spreadsheets - 67/68", by application, under description "Shipping Tie Down Plates".
Jim