On the back of the block there are 4 cup plugs, 3 small that plug the lifter galleys ( center main galley and right and left) and 1 larger cam plug. You can replace the 3 smaller plugs with threaded plugs but of course the engine needs to come out. Threaded plugs used there became standard in 69 I believe. Anything earlier will have pressed in cup plugs stock. The larger rear cam plug should be installed with the cup side facing in towards the rear of the cam. I also run a bead of sealer around the outer edge of the cam plug. It's common for the smaller cup plugs to weep after time. One of those things. It could also be the rear main seal leaking possibly. Or all of the above. The 390 is a top oiler.