I had one leaking in the back of the right head.
Dad looked at it, got out the drill and drill bit and drilled a hole in the firewall behind the head. Took a punch and knocked it in turning so we could get it out with plyers. I probably put in a rubber plug.
As far as the ones in the bellhousing area, pulling the trans or engine is only option, pull the engine change them all plus any other gaskets
It has been years since I have had to change one. One of the hardest I ever changed was the one in the right head of my brothers 69 383 GTS Dart.
Was on my way to school and pulled out on US 40 and stood on it, white smoke started rolling out the back. Antifreeze was hitting the hot exhaust, so I took the guy riding with me back home so he could get his car, I skipped school that day ( hey I had 2.5 hours auto mech, so I was still doing what i would have at school) and spent the whole day changing the stupid freeze plug. Mom and Dad had already went to work and I got it done before they got home.