Just some advice, I am currently restoring the opel in my .sig, the engine has not been started since '75.. It was seized as well, I tried all the tricks with oil in the plug holes etc, nothing worked. So I removed the head, and what do I find? Three perfectly good cylinders and one cylinder filled with debris, rust buildups on the cylinder walls etc.. Depending on which valves were open on your engine when it was put away 25 years ago, you will need to fix some of the cylinders, and definately remove the heads.
