I found letters and numbers like so:
C27-9
GM4T
PASS
I'm guessing it was produced in March (C) 27th day of 69?
Anything else from the other numbers?

The casting number should be inside the valve cover. That will help allot. Pray for 840 as they are the most valuable. Not positive about the 4T, but I think the T is for Tonawanda engine plant. C 27 9 is for march 27, 1969 as you suspected.