Blew rear freeze plug on 408

You're not going to push a freeze plug out because of pressure building in the system unless the freeze plug was improperly installed and of story that's it.
First and foremost make sure it's in pressed in enough, especially if you're the type to use rvt..because if it's not set in right with enough inference..the rtv will actually help it pop out.

Been there, done that. My buddy. who is a pretty bright dude...helped pop plugs into my block and didnt set the rear enough...blew out on the first hard shift all over , clutch,pp.
Live and learn..and some things you only do yourself.

The one I blew out on the 383 was in a 71 Chrysler Newport and it was in 1976 so it was factory installed and only 5 years old. I don't think it was installed incorrectly, It came out at WOT wound tight so it was a head gasket leak that pressurized the cooling system. It was my mom's car and my dad kicked my *** every day till I finished replacing both head gaskets.