Stock 400 heads are the same casting as 350 heads. At the factory they drill four steam holes in the deck of the head to help try and cool the 400 better. 400 heads can be used on 350 engines with out any problems. If you want to install 350 heads on 400 engines then just drill the steam holes in the heads. Use the 400 gasket as a template so you know where to drill. Make sure you look at a 400 head first so you know the angle at which the holes are drilled in the head.