the walls in the front studs arent as thick as the centre stud walls hence you only stud the centre 6,you then can clamp the head down with a thicker stud and nut kit and use std head bolts on front and rear studs...
you can go 10 long studs when using nikasil liners and using a grp a water pump to clear the front studs...
not the best description i know but im sure someone can explain better than me but thats the basics

cheers danny