I believe the serial number for the motor will be found hust above the oil pan on the passenger side. They are hard to read. A 1956 GMC could have left the factory with a 316 cubic inch pontiac motor. It is possible that the motor was changed over the years.
Lets see if we can narrow it down by description.
How many bolts hold the water pump on?
Are there any tubes going into the front of the heads from the timing cover housing/ water pump housing?
What does it have for carburetion?
I went to
http://pontiacsafari.com/ and looked up some of the numbers in the garage secton. The only number I could find was for the cylinder heads and it was consistant with a 56 316 engine.
