Just Purchased Virgin 340 Short - What to do with Pistons

Mattax said
That's the casting number.
View attachment 1715527541
On the other side of the Block, will be the casting date and time.

Engine number, including VIN or partial VIN was stamped onto a pads.
Look on the front, below the heads.
View attachment 1715527547
and above the pan rail on the right.
View attachment 1715527551

Some examples with decoding comments here:
Mopar Lost and Found

Well that's a 69 block
But I do not believe that is factory paint and it appears that those freeze plugs have been replaced.
My guess is that that engine has been apart before. Hence the different pistons.