Ok I finally got the cranks off. Thanks for the helpful advice. Once I had confirmed the removal process, I was able to get more aggressive with them. Of course, it also helped that I didn't have to worry about damaging the frame. I ended up just loosening the crank bolts as is, but I used a picnic table for some leverage, without which I don't think I would have been successful. I placed the frame on the bench part and used the tabletop to brace the opposite crankarm so I could really put some muscle and weight into the crank I was working on. They both eventually gave way. The bottom bracket was also a chore to remove, but I finally extracted it as well. These cranks may not look very pretty anymore, but they still seem perfectly functional.