Check out the ARP 100-7101 7/16" studs. I'm in the same boat as you with Perf RPM heads and a set of 7/16" roller rockers. I picked up a set of these ARP studs to work with the new rockers. I haven't swapped them all but have tried threading one in hand tight and it's just the ticket.
The new studs are machined flat across the top of the stud to work with locks too - unlike the cheepo 3/8" studs that the Edelbrock heads came with.
My Edelbrock heads are 7/16" threads for the studs. It's only the upper portion of the original stud that is 3/8".