I agree about cleaning the bore above the piston, also if you can tap it down about a 1/4 inch and clean above where the rings have been sitting and then drive them back out it can be pretty easy even on a really bad motor.

Another thing that worked for me on a really bad 340 was to take all the rod caps off first and pull the crank, then put the caps back on the rods and hit them with a dead blow hammer.

Really good penatrent is important, let em soak for a good long time.

On a couple of the pistons I ended up drilling holes and busting them out a little at a time.

The worst motor I did was that 72 340 and it had cast pistons and they are tougher than you would think when you are just trying to bust them out, the pesky steel strut cast into them is a real pain

Warning I have never tried this on a car.
I am learning blacksmithing.
When a piece of metal is heated in a forge the carbon content comes to the surface as forms an ultra hard layer.
So hard it destroy hand files easily.
If soaked in vinegar you can wipe the layer off easily with a towel.
Leave it to long and it will get very fine rust.