Depends on the year; earlier heads are coarse thread 5/16", later (EFI) are 1/4-20 by about 1/2". Not sure on the length; I used studs on mine with nuts to make valve cover gasket changes easier.
1985 F150 4X4 300 I6 4-Speed
1970 Torino Cobra "Twister Special" 429CJ 4-Speed
1965 Mercury Comet Caliente Convertible 331 5-Speed