Thanks for the quick reply! I lubricated the plug, got a socket the right size, and tried hammering it in. All that did was put a circular dent in the plug. Being in the trunk, getting a good swing with the hammer wasn't happening. I suspect I need a press. Anyhow, the body plug kit had some other plugs the same size that were less substantial, and I was able to press them in with my thumb. It's good to go, but my car has lots of compromises in it like that and it bugs me.
I've had the car for 30 years and have never been able to find someone nearby to work on cars with, where we help each other. The local mopar club just talks about fundraising and cruising. There are muscle cars in the neighborhood, but all of them were bought with a check, not sweat. Grumble, grumble.
I struggled with the brakes for a long time, and finally gave up and took it to a shop. Now the brakes are solid which makes me happy. The mechanic told me I simply had not torqued the brake line fittings down hard enough. There's a lot to these cars that ain't in the manuals.
But my 400hp heavily modified 340 is back on the road again, after decades as a garage queen. People go ape when they see it roaring by.