Travis is right, the best thing to do is pull the heads and have a machine shop pull the studs out, and machine the heads for screw in studs. I would also mill the stud bosses for use with pushrod guide plates as long as you are doing this. You'll want to run hardened pushrods afterwards. I have milled the stud bosses myself and it's qquite easy if you have access to a good mill. It's also very easy to drill and tap the heads yourself for screw in studs. But unless you have experience and a good set of machine tools leave it to a good shop. It doesn't cost much. I would also like to recommend you use ARP studs not GM. ARP have more clearance in the radius base and are longer overall. They also have more threads that run further down the stud. This is important in case the machine shop doesn't remove quite enough material it will allow you to adjust for the correct valve train geometry. Dorman and Summit both offer screw in studs, but Summit's don't have as much clearance in the radius area and Dormanhave a base nut that is quite thick which will take away from the correct geometry you're shooting for.