According to the "1969 Mustang Chassis Assembly Manual" (AM0030) page 25, the nut is 34661-S2, 5/16-18 Hex with an external tooth lock washer, the same as the nut used from that bracket (C9ZF-9A627-C) to the air cleaner housing base.
It is documented in this forums library in topic "Mustang Hardware Spreadsheet 1969-70", file "MHW69-70AP06" which lists hardware by application, "Air Cleaner Bracket,Ram Air-Intake . ." to both manifold and housing.
I suggest you get a set of the Osborn Mustang Assembly Manuals for your year car.
Jim