6066 (1960-1966) GMC Truck Club - Removing stuck pistons

I recently bought a 401M long block that was never used. It ended up outside with no covering except a hub cap over the intake. It was very stuck. I pulled the heads and found that only two cylinders were in trouble and was able to remove the other four along with the crank. Pistons were brand new standard size. Cylinders where sleeved. I cleaned the two rusty cylinders as good as I could and rotated the block so these were level. I covered the pistons with muriatic acid which is available at Home Depot for cleaning swimming pools. I soaked for an hour and then cleaned everything up. One piston came out and looked reusable. The other needed and additional hour and it came out. I'm not seeing anything wrong with the pistons but I haven't removed the rings and inspected the lands yet. I have them soaking in cleaner right now. Both sleeves need to be replaced. I think Ill try to locate two new pistons but I wouldn't be surprised if these two could be reused.