I have two 1968 Pontiac 400 blocks and was checking the VIN numbers on both. One has a very tiny VIN number ( I measured 1/8" tall with calipers) and the other is about .2" tall. I did not check at what plant the blocks were stamped. Did Pontiac use VIN stamps of different sizes?

John it would help if you had some pictures of your two blocks.

Pontiac cast all of their V8 blocks out of the Pontiac Foundry in Pontiac Michigan. So all the casting lettering should be consistent, ditto any stamped numbers at the engine plant (also in Pontiac) unless the motor was pulled for a special order or engineering experiment/engineering order car. So in some very extraordinarily rare cases, the assembly plant could have re-stamped the engine. Which I doubt is the case here.

Are you trying to determine if one of these blocks is the numbers matching block to your car? If so, refer to the chart below. The numbers you are looking for are located on the front of the block, on the passengers side, at the bottom near the timing cover.  It should start with "2" for Pontiac, the next letter indicates the assembly plant the car was built at, and the last 6 digits should match the last six digits of your vehicles VIN #.

 

Here are a couple of videos that might help identify your blocks.

 

 

There are two stamp pads on the front of the engine block. On one side you have a nine digit number that starts with a "2" (for Pontiac) with the remaining eight digits being the last eight numbers of the cars VIN. This is a pic from my late great GTO...

You can see they match the last eight numbers of the VIN (highlighted in yellow)...

 

 

The pad on the other side of the front of the block has the engine production number and a two letter code that identifies the engine and the transmission (manual or automatic). Mine was a "WS" code for 400 HO/manual...