In my experience, some studs do pull out just due to old age, and a slightly loose press fit from the factory. One thing that will start the process, on occasion, is a loose valve adjustment that allows the lifter to hammer on the valve stem. Something about the shock of taking up the looseness starts the stud moving.
Yes, by all means, get the machine shop to install thread-in studs on all the valves while they're in there. The most time-consuming part of the setup is getting the head aligned where all the stud bosses are pointing straight up. Once that's done, the drill/tap procedure goes pretty fast. You can find hundreds of sets of screw-in studs on Ebay. Just get a 3/8" set for a smallblock V-8, and you'll end up with 4 spares. The studs for the 6-cylinder are virtually identical to the V-8 type, and you won't have to pay the inflated prices just because it says "6-cylinder" on the package. Unless you're planning to go racing, get the type without the built-in nut, and all your machinist will have to do is pull the old studs and run a 7/16X14 tap into the holes. No milling! Use a drop of red LocTite on the studs as they're installed, and they'll stay there forever!
Jerry
