Usually, frost plugs only come out when water freezes in the block. And that can be tricky, as sometimes the block suffers.
Frost plugs are difficult to install even with the engine out on a stand. They have to be started square in the block and pounded in with a hammer hitting it square on.
Do a little research and find out if any frost plugs can be replaced with a block heater, as it will be a lot easier than trying to pound in a frost plug. Only certain locations will accept the freeze plug due to the proximity of the cylinder wall, but if you got them cheap enough, you could remove the heating element which is what hits the cylinder wall.
Get a mirror back there with good lighting and see how many plugs you need, and look for anything that looks like a crack between the plug holes. Even if it cracked, it can be repaired if you are willing to take the engine out.
Have a good look at removing the y-pipe and lowering the subframe enough to get in there to take a whack at the plugs. You may have to remove the subframe and support the engine from the top, which makes it risky if you are the guy under there, so have a redundant support system in place as well.
Good luck.
