Fig. 43.1
If you find a block with three freeze plugs on the side as seen in Fig. 43.1 you can be sure it is a 400-CID. Most all 400-CID blocks with three freeze plugs had four bolt mains.
Fig. 43.2
But not all 400-CID blocks had the three freeze plugs some were cast like all the other small-blocks with only two freeze plugs as seen in Fig. 43.2. Most 400's with two freeze plugs had only two bolt mains.
Fig. 43.3.
There is also 400-CID blocks with the third freeze plug boss but no hole as seen in Fig. 43.3. Note as with most things remember a two freeze plug block could have 4-bolt mains nothing is written in cast iron.