She's using brass, which won't rust, but a lot of early Hemi guys use JB Weld to seal the plugs on those blocks. They actually have convex welch plugs that install and must be flattened a little with a hammer to wedge them in.
yes i had to do some of those on a '58 cadillac last year. there was a step in the machined hole so the plug goes in as far as the step then you give it a clout in the middle to flatten/expand it as you say.
neil.