In the interest of accuracy and brevity, the 383 and 400 will be referred to as B engines while 413 and 440 engines will be referred to as RB engines
426 engines are also RB but a 426 should be very easy to distinguish due to the huge heads with the spark plug wire going through the middle of the valve cover.
Many times I've asked, and been asked, "Is that a B or an RB, and how do you tell the difference?". Of course there is the text book answer, "Look for the number stamp pad located here and there." The only trouble with this method is that those little pads can be VERY hard to see, especially on an engine that is not exactly clean, not to mention the fact that on A/C cars, the RB's number pad is buried under the A/C compressor and can be extremely difficult to find. Because I have a few cars and more than one example of each engine, I've noticed a far easier way of determining if a big block is in fact a B or a RB.
We will be focusing on where the driver's side head meets the block, AKA the "deck". 400 Engines are known as "B" engines and 440s are "RB" engines. "RB" = "Raised Block" (or raise deck) This raised deck is what gives the 440 the longer stroke than the 400 and therefore those 40 more cubes. This raised deck can be seen on the front of the engine where the head meets the block.
As you can see on the left, the B has a lower deck than the RB on the right. The RB also has some kind of bead or web on the corner.
I find this method reliable and far easier than trying to find a number pad that may be buried under an A/C compressor. I hope this helps others.