Couple of thoughts. You sure the heads off the 389 or are they off a different engine? Maybe the heads were using hollow studs that oiled through the studs not the push rods. I don't think any of the production heads had screw in studs in 1965, but who knows. Could be the studs were pulling out and someone tapped them for screw in. If some of the studs are still press in and some screw in I would think you'd want to have the shop tap the whole works for screw in. The standard Pontiac rocker stud since the mid fifties for hydraulic lifters had a shoulder on them just below the threads, the nuts have a matching bevel. The nuts were just run down to the shoulder and torqued to 20 foot pounds to hold the rockers on. No need to adjust the lifter pre-load. You can use big block Chebby studs which have straight shafts with no taper, but you will have to use some locking nuts and set the pre load, that's no big deal. If your machine shop is going to put on Chebby style it will have to be Big block studs as they are shorter, to clear the rocker covers. One can use the small block ones and cut then down to the right height, but may as well use the correct length from the start Just my opinion, I'm not an expert. .
