I have the engine of my '75 300D on a stand, and have a block heater to install. The heater is threaded, with a threaded bushing for a larger hole. The plugs are not the "soft" freeze plugs as I have seen on other engines, they appear to be cast iron threaded plugs, with a 19mm Allen socket inset in them. There are 4 on the passenger side. The driver side has two smaller ones. I cannot get them to budge with a breaker bar and a cheater bar!

Are these supposed to be there, and if so, how do they pop out if the block freezes? Are they threaded, or is there some trick I am supposed to know?