Pat
I just answered this same question for a guy about a 41 Olds.
On the 40 the serial number is on a tag attached to the top of the frame at the left front. You should be able to see it by opening the hood and looking down on the frame rail. If the tag is missing, the other number you can use for registration is the motor number. Its on the front of the block above the water pump and on a tag attached to floorboard near the right front door. Motor number prefixes are G for 6 cylinder and L for 8 cylinder.