I have a 216 in my 46 2 ton that is seized and a 235 in pretty good condition also seized, what is the best thing to use to unfreeze the piston.

Also the 235 was rebuilt, the guy that gave it to me told me it was newly rebuilt but has been sitting so it needed work, when I torn into it all the parts are new and the 030 stamp is perfect on the piston and the rocker arm still has identification paint on it, but the problem is the block was cracked and re-welded, was this common? Can a block be repaired correctly or was the person putting new parts in a worthless block? I was just trying to figure out why? And would like to know if its safe?

Thank you for any help,
Josh