I just replaced press in studs with screw in ones on Pontiac
V-8. I had to buy a special stud since the holes were 3/8" all ready, the new studs had 7/16" threads.
The holes were at the right diameter to tap so no drilling was required. To align the tap, I used a harden nut held down with a body shim (long legged shim used to align sheet metal) that was bolted to stud next to it. It worked really well, and with lots of tapping fluid, I did all 16 with one tap. The nut worked great in holding the tap straight and was a easy to use.
I used a older tap with the end cut off to finish out the bottom thread or two. I still ended up cutting off a few threads from the studs since they were so long. The local machine shop told me about the nut idea. They had a tool made that was nothing more then a thick piece of steel with one through hole for bolting down and one threaded hole for the tap.
Even if the holes are not 100% straight, the rocker pivot will compinsate for it, as long as you are fairly close.
Joe