Here is what I found on plugging the oil supply to the rocker arm studs," With the changes in rocker arm oiling came new head gaskets. Those for 1964 and earlier had a hole for the passage that supplied oil to the galley in the heads to feed the studs. Later gaskets didn’t have that hole. When installing 1965 and later heads on an earlier block one common practice was to use a later head gasket without the oil hole. However, many gasket companies have consolidated part numbers, the result being some applications have been expanded so to broaden the application later head gaskets may have the extra hole. In that case another option is to plug the supply hole in the block by drilling and tapping it for a 1/8-inch pipe plug."
On the rocker arm studs, "Another unique feature is the valvetrain. Prior to 1965, Pontiac’s stamped steel rocker arms were lubricated by oil delivered through the heads that lead to passages in the rocker studs—holes in the sides of the studs delivered lubricant to the rocker balls. We opted to install screw-in studs from COMP Cams. Another reason for changing the studs was adjustability. In stock form the rocker arm nuts bottom out on the shouldered stud and are torqued to remain tight—any change in deck height, head thickness, or valve stem height alters rocker arm geometry and there is no way to correct it. The cure is screw in studs with locknuts, and in some cases custom-length pushrods are also required."
My guess would also be that if you installed screw-in rocker studs, they are solid and will not allow oil through, thus shutting off the oil supply to the heads the same as installing the 1/8" pipe plug. You also want to change your rocker arms if they do not have the oiling holes in them for pushrod oiling.
ARP number-290-7201 rocker stud kit is specifically designed for converting an original pressed-stud cylinder head to threaded studs. The ARP studs feature a typical 7/16-inch upper with a 1/2-20 base. The base threads are purposely left slightly long and must sometimes be trimmed to fit. Overall stud height: 2.000 in. * Screw in thread depth: .1.025 in. * Adjuster thread depth: 1.05 in. * Shank diameter: 7/16 in. * Base diameter: 1/2 in. coarse * With roller rockers, and girdles * Not to be used with OEM style self locking nuts. NOTE- I believe you may have to spot face the stud pedestals flat.
However, these are offered on Ebay and state they can be used without any mods to the pedestal. Might be the way to go: https://www.ebay.com/itm/Pontiac-GTO...-/350433783872