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The '72 will have a 6 bolt crank. The other two will have 8-bolt cranks.
Rods and pistons are different between the two, but blocks are quite similar.
I've heard through the grapevine that castings are better in later years, but any year can give you a dandy or a dud. If you're overboring, your machine shop can do a sonic check of the cylinder wall thickness for not too much, and that might help you choose the best block. The blocks are similar enough that the condition and/or any built-in defects or inclusions will trump any difference from year to year.
The fact that one of the engines happens to be powering a vehicle right now, might affect your ability to rebuild it without having to walk for extended periods. It's not like the '74 is a magical super-desirable engine. Well, it's a great place to start, but so is a '75. So, use one of the two that are out on the bench already.
The head is a matter of taste, and the archives are full of discussions thereon. This topic has already been flogged to death. Personally, I'd use the '75.
Yes, the cranks are interchangeable between blocks. Main bearings are the same.
Rods are not interchangeable between cranks, and pistons are not interchangeable between rods.
But, you can take the crank/rods/pistons from a 6-bolt motor and put the whole ensemble in a block that originally had an 8-bolt or vice versa.
The 6 bolt is, frankly, better. Mainly because the rods are lighter, by quite a lot. All things being equal, choose the 6-bolt.
The 8 bolt, however, often has better availability of oversize pistons. So, you may well go with it for that reason.
You have choices!
Basically, choose the best block based on what your machinist tells you about the condition of the blocks you have.
Choose the best crank based on what you can find for pistons.
Use the rods that came with the crank of choice.
Enjoy the process.
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