If a engine still has antifreeze in it and (not plain water) they are most of the time good, as far as cracking from the cold.
If it turns over and there is oil in it and it looks like nobody messed with it, then it's a pretty good chance the engine is fine.
You could always try to get the lowest price on the 400 and then offer to buy both engines at en even lower price.
If the yard says yes to $100 for the 400 then possibly they will say yes to $150 for both engines.