The 4V block has 4-bolt mains, whereas the 2V has 2-bolt mains. But the bottom end is so strong that rarely does a cap let go. I found several internet sites with examples of engines that had swallowed valves and "locked up" with bent connecting rods, holes out the side of the block, but the crank was still within specification and the caps were in place.
The 400C was made in 1971 to compete with Chevy's 396. The 351C block was stretched (taller deck height) to accommodate the longer stroke (4"). The taller deck height means the intake is bigger than the 351C and the block itself is wider. The 400 crank journals are larger than the 351-C, and the connecting rods and push rods are longer that the 351-C. In 1973 the 351-C and 400C were "merged" into the 351M/400 engine. Thus the 351M has the same block attributes as the 400C, being taller and wider than the 351-C. The 351M heads are the open chamber 2V type. They are completely interchangeable with the 351C and 400C 2V heads and, since they were made after leaded gas was eliminated, they have the hardened valve seats for longer life. The 351M/400 crank is has the larger journals, like the 400C. So it is easy to turn a 351M into a 400; just change the crank and rods. But the few parts that are interchangeable between the 400C or 351M and the 351-C include the heads, cam, distributor, pistons, water pump and valve train. The details behind these general statements can be seen in the specifications in the linked web pages. Have fun reading them. I tried to put in more than anybody really wants to know about a 351-C. I also have lots of comparisons to other Ford engines, such as the 351W, 351M/400, 390, 460.
One other thing to keep in mind is bell housing bolt patterns. The 289 (starting in 1965), 302, 351-W and 351-C all use the "small block" 6-bolt bell housing pattern. The 400, 351M/400, 390, 427, 428 and 429 all use the "big block" bell housing, which are heavier, more difficult to find and more expensive than the "small block" size.
Identify a 351-C Donor Car
Look only at the 1970-1974 cars. The full sized Ford, Mercury and Thunderbirds could have a 351-C in 1972-1974, but my books don't show them as being used in these models in 1970-1971. The lighter cars, Mustang, Falcon, Comet and Maverick, could have a 351-C, but the Mustangs will be snatched up and picked over quickly, and it just wasn't popular to put powerhouse engines in "economy" class cars such as the Falcon, Maverick or Comet. They usually had a 6 cylinder, 289 or 302. Look at the mid-sized cars: Fairlane, Torino, Cougar and Montego in all years, 1970-1974.
Be careful, however. If you look at a 1975, you may accidentally find a 351/400M. The 400-C, found as early as 1972, looks a lot like a 351-C. Start with the vehicle certification tag placed in the driver's door jam to find the year and engine type. They won't tell you if you have found a Windsor or Cleveland, but at least you'll have the correct year and displacement. Learn to distinguish between Windsors and Clevelands here.
The vehicle certification label will identify which engine is in the car. Use the engine identification tag attached to the coil mount to confirm the engine. Be careful, however, sometimes these get changes to try to deceive people. The picture below shows the tag's position.
1 Windsor and Cleveland versions of the 2V were used interchangeably
from 1970 through 1974. In 1975, the 400C and 351M were merged so only the
351M/400 was used.