Dan,
There a some Canadian GM casting codes at the
bottom of this post. Maybe someone will post a better source of Canadian codes - I do not recall anyone making/posting a list?
You can determine if your truck is a high oil-pressure 235/261 or a low-pressure 216/235 by a few characteristics.
low pressure:
- 15 head bolts
- valve cover attaches to head with two bolts
- the is a small triangular sheet metal plate on the block in the middle of the manifold side of the block
- water pump body extends above the top of the block
high pressure:
- 18 head bolts
- valve cover attaches to head with 4 machine screws
- water pump body is completely below the head
You can usually determine the year of the engine from
date casting codes. 
