Ok, now that makes perfect sense as a 7/16" (0.4375") pilot hole for the 1/4" NPT thread would be just under the minor diameter of 0.442". But that's not possible in the forward oil galley holes as they are already 1/2" bore.
Based on that, a 3/8" NPT screw in plug would be better as it has a minor diameter of 0.578", above the 1/2" bore, so it could be tapped properly to a full thread height.
Maybe the stress on this thread is actually quite low, so over the years if everyone has just been using 1/4" NPT without any problems then it's probably fine to do just that.
I've seen mentioned that some of the Shelby and BOSS blocks had screw in plugs from the start of their life. Out of interest, does anyone know if they were retrofitted afterwards or if extra material was left in the block? You could easily tell if you had one of these blocks by just measuring inside the threaded hole. If it's 0.50" then it will be retrofitted, if it's 0.44" then they must have left extra material in.