Looks like a 1970 Block (1382201)
Heads look like 1968 ((1233472)
Intake manifold, I am not sure off, might be mid 70's.
The stamped number on the block is the VIN number derivative.
The heads have coolant ports on both ends, BUT, coolant only flows across the front of the intake. The reason for the coolant holes on both ends is because the cylinder heads can be installed on either side of the engine, they are reversible, no separate left and right side head. The coolant holes in the back are blocked by the intake manifold with the exception of some late 60's intakes having the heater control valve screw in back there.
Have a look over at www.v8buick.com. You'll find a lot of information and help there.
1970 GS455 Stage1 Race weight 4025 lbs.
TSP 470, 602 HP, 589 TQ
MT headers, Gear Vendors Overdrive
Best E.T. 11.54 Best MPH, 116.06
1998 Riviera SC3800