If a junk yard is selling the engines it is a good bet that they are not in a running situation (i.e. in a vehicle with fuel and spark). Of course they will say "they run great", but I doubt they have ever seen them run. I would, at a minimum, have the cylinders compression tested. Will a few variations, this should tell you if the rings/head/valves are any good. It will also warn you if the engine is seized. If these are rebuild cores for you, any ring/valve issues may not be a big problem. If you are actually going to use the engine used, they are more critical. Good luck!!

JD