Any of that era LA motor (as compared to the later model Magnum small blocks, or the earlier "A" motors) has the size of the motor cast into the block on the left side of the block near where the starter was located, (*318* ,* 340* , or* 360*) in raised numbers. Its pretty easy to identify the block by the raised cast in numbers. Some of those raised cast in numbers are pretty hard to read as well, but its pretty easy to determine the difference between an 18, a 40 or a 60.
The numbers stamped on the pad were numbers stamped at the completion of the machining process, and many of those were really low quality stampings (particle numbers, weak nearly impossible to read stampings) are not unheard of.
If the block has been cut, some or most of that stamping may be gone. With this many years gone by, its possible the motor is not the original to the car, it could be a different motor from the same time frame, or it could have even been a replacement block someone stamped themselves at some point. I'd be pretty suspect about a motor that old with that much shiny paint, unless the rest of the car is also pristine original.

Edit: As I read the casting date, I see a casting date of 11 of 71 on the 2nd shift. Should have been a 72 motor.