I had a very rusty engine last summer that I went through this with. It soaked for 3-4 months in and out of the molasses bath as parts SLOWLY came off. After everything was off the engine except the crank, rods, and pistons I tried everything to get the pistons out. A couple did come out with the hammer and a heavy rod, driving them out the top. In the end after several weeks of trying I ended up pulling them out. I drilled a 5/8 hole on each side of the wrist pin, opposite of each other on the end of the pin and dropped a 1/2 wire rope cable down around the pin. A cable clamp to tie the cable together, a block of wood, a large pipe and a 20 to short bottle jack pulled them out of the holes. I put pressure on 1 at a time with the jack, heated the cylinder the best I could through the soft plug holes and used lots of ATF/acetone mix. I would give the jack a couple of pumps each morning before I would leave for work, come home and put some heat on it and give the jack another pump or two, what ever it would take. Some holes took 2-3 days before they came out, but every day you could tell they were moving. I did not hurt the bores in the block.