Sometimes the valves need to be rotated or spun to break up the rust. I know this because......
I had a 1951 Pontiac with several stuck valves and I soaked them, beat them down with a rubber mallet, etc, but they continued to get stuck in the open position.
Since I had the head off, as you do, I used a pair of pliers that had rubber-lined jaws to grab the valve head and rotate the valve (while squirting lube down the stem) This fixed the sticking problem and caused no damage to the valves.
Also, while you have the valve covers off, make sure you clean out the area(s) where the oil pools (it's the area below the springs). I'll bet if you stick your finger in there you will find that the pools are loaded with sludge - which is normal for an old engine that was never rebuilt or taken apart. Just clean them out and wipe out the residue.
Same thing when you drop the oil pan - clean out the sludge and clean the pickup screen, too.