remove the header before tapping the hole to 7/16 or helicoil

if you dont get it straight,and mess it up,the head will need to be replaced

to helicoil it you need the correct size drill bit with the helicoil kit

the kit has a tap and coils and the coil install tool

remove header,drill to size,tap hole,install helicoil with some rtv on the outside of the coil,put header back on and then use the stock size bolt with rtv on it..LET THE RTV CURE OVER NIGHT!...add water to rad and run engine and check for leaks

or like said,use a 7/16 tap and use a larger than stock bolt after drilling the header flange for the bigr bolt