The carriage bolts are p/n 378535-S2 per the 69 Chassis Manual, and the nuts are 33771-S2, same reference. The nut "may" be either a round top or flat top style (different suppliers?), but look to be grade 5 UBS type in either case. They are shown as phosphate, but may have been zinc plated They are both referenced in the Library Section of this forum under "Mustang Hardware Spreadsheet 69-70 - file MHW69-70AP02.xls. From there, sources may be AMK.
Jim