For the record, I ordered and received the Vauxhall Opel 2.5L studs (ARP Part No. 209-4702 ARP. After taking them to my engine builder, we noticed because the studs are undercut, they only threaded into the block down to where the shoulder of where they are under cut. Therefore, with the head in place, the thread for the nut stopped about 10mm short of the seating face of the head. It would have required the use of several fat washers to make up the space which was a very unacceptable solution.
I have since returned these in exchange for the Toyota Supra head stud kit, ARP# 203-4205 which are not undercut studs.
Ironically, it seems those who have used the Opel head stud kit as I ordered originally, received studs that weren't undercut - I've no idea how this seems to be the case when the part numbers are the same and it seems very strange indeed. Regardless, the Supra head studs have been used in the past with great success so I believe all should be good regardless.
The down side was it's cost me an extra $100 in freight to return the Opel kit and have the Supra kit sent out.