There are two types of screw-in studs, the ones with a nut built in, and ones that just have a 7/16" thread on the cylinder head end, and a 3/8" thread for the rocker arm nut. If you use the type with the integral nut, the stud boss needs to be milled down about 1/4" or so before it's threaded. Either way, you're going to make a lot of shavings, so it's best to pull the head and have a machine shop do ther work. It's also very likely you'll tap the thread crooked unless the head is in a seat/guide machine, or at least a drill press while the threading is being done.
An alternative to threaded studs is to drive the press-fit stud back down to its normal height, and drill a 1/8" hole through the stud boss a little off-center, just barely putting a nick in thre side of the stud. Then drive a 1/8"X1" spring pin (roll pin) through the hole, locking the stud into place. If you're careful, and pack a rag around the stud boss, the drilling can be done without removing the head. Do a THOROUGH cleanup afterwards to keep cuttings from getting into the crankcase.
It's better to do any machining, drilling or tapping with the head off, though! You might get away with drilling and pinning one stud with a minimum of risk, though.
Jerry
