The step in my block is consistently from 16mm (where the plug sits) down to 12 mm for the oil gallery itself.
A 1/2-20 tap goes in but the resulting thread is pretty shallow, only about 0.007" thread overlap on a standard bolt, compared to 0.015" with a standard tap dill.
The loads are small so likely not an issue except the plug on the front where the drilling to the aux shaft bearing is close to the end of the gallery, leaving room for only two threads.
Since that one is under a cover, maybe setting the screwed plug so that is flush with the face of the block would allow the aluminum cap to retain it.
I think I'll do that and lock the plug in place with Loctite 620.
The other two ( stupid question of the day - are there only three plugs in the block?) have a lot more thread so strength won't be an issue.