[Dominik]

Look at your bolts, they should be 3/8" NC, about 3/4 - 1" long. The studs would be 3/8"NC inside the head and 1/4" NF (with nuts) to hold the header. Total length must cover your 3/8" threaded part in the head, plus shank in flange thickness, plus the 1/4" thread with the nuts.

In doubt, order 2 sets... Summit and Jegs should carry them for small money. For headers you won't need highest grade bolts, but ARP makes the nicest set :-) (usd$70?)

You can also mix studs and bolts if you spaces are tight and the header won't fit over the stud. Or, insert the stud later. Hand tight will suffice.

Sunny greetings from South Africa!