72 340 short block

If you are looking at just "surface" rust as in a light coat of it with very little to no pitting in the metal itself...I would just clean up the cylinders.
I have been doing it for years,you would be surprised to see some engines I have brought back lol.
I had and engine that sat in the weeds for years with no spark plugs and valve covers off and exhaust manifolds off,pistons were locked from rust,pulled it all apart and had pretty bad rust but I brought it up to par and it didnt smoke or use a drop of oil.
It all just depends on if you know what your doing I guess.
I normally take some light emory cloth strips and knock down the rust until its kind of dusty smooth I guess you would say.
I then hone it out, I have used bead hones and block hones but the bead style is harder to find and if you work a spot to long you can "egg shape" the cylinder slightly.
A nice block hone with some good sharp stones and some WD-40 will make them look like brand new after knocking the rust off with emory.
Load it up on the drill and start off slow and spritsing it with WD-40 while its rotating and keep working your way back and forth (don't hold one spot or you can create a ridge).
As you go along you feel less drag and visibly notice the difference in the cylinder and you can gradually speed up the drill and towards the end you will be going full speed and in and out of the cylinder non stop.

The end result should be a nice glass textured cylinder.
You can atleast try this first and see what you think and see if you can save the cylinders.
It sure is alot cheaper then having to have the block bored and sonic tested and buying new pistons and rings and everything else....when you could just be looking at what it will cost you to hone it and toss rings on the pistons you have already! Something to think about any way ,good luck!