The bolts are 8mm.......you probably can get away with some 5/16 bolts too.
You will need the actual valve cover to make the spark plug cover work.
Hector
2003 Rally Red Mitsubishi Evolution VIII
257HP/259TQ
2005 Lapis Blue Mazda 6s
RET: 00 Cabernet Red Ford Contour Zetec ATX SUPERCHARGED
160HP/141TQ