Here are some pictures of my NOS 41/42 Mercury / WWII block. Notice that it has a factory relief, has a raised intake surface and has the round center water hole. It does NOT have core plugs in the pan rail and comes stock with a 3 3/16 bore (239 cubic inches).
The Ford blocks will have a trapezoid center water hole and will be 3 1/16 bore.
I'll include pictures of the AFTER state of the block - as I've heavily modified it for performance. You'll see heavy porting, full relief, large valves, large ports, etc..
Stock:
image.jpg
Heavily Modified:
BlockPrep4.jpg
DeckComplete.jpg