Looks like your Grandpa wasn't shy on the options. Unless you know for sure, there is always the possibility that the engine is not the factory engine that came with the truck.
In this case, the easiest way to id the block is to locate the engine VIN pad (front passenger side of the block) located at the top of the deck where the cylinder head spends it time.
I'm sure there are several good sites that provide info on these numbers/letters, but the one I have found most abundant with info is:
http://www.nastyz28.com/chevy-engine-code-stampings.php
The numbers on the block ID pad will help you identify where the engine was installed in the vehicle, what type of vehicle it is (C20 pickup) and there will be a series of 2 or 3 digit letters known as the suffix code. Those are the letters you'll be looking up in the above link.
Hopefully this helps. The info I have found has been pretty accurate with the exception of my '67 283 block that is original to my truck. Running the casting number it comes up as a '67 283 with some notes saying that it was replaced with the 307 in '68. But when I look up the suffix code, the resources identify it as a 307. Deciding factor was the good old fashion visual check which was the PCV valve at the back of the lifter gallery, and the tall oil filler tube at the front of the intake manifold (pretty sure 307's didn't have these).
Anyways, check out that website for further confirmation on your engine block's history. And good luck on your truck.