what thread size are the exhaust bolt holes on a stock small block LA series head?

I need to order a helicoil to fix a striped hole and want to make sure I get the right size.

I am also buying new bolts and or studs, when I look them up in the Summit catalouge, the bolts are 5/16 and the studs are 3/8...what gives

Thanks,

Jas

5/16-18