Andy,
I have used the dished core plugs for the front and rear holes, not the cup shaped plugs. You want to install them with the dished part rounded OUTWARD, and the plug will just about drop into the boss in the block. Then you center a large drift on the plug and give it a coupla taps with a hammer to flatten it out slightly, which makes it seal in the block, as Drew mentioned above. Oh, and I always use a bit of the old-style Permatex sealer on them too.
Hoyt
