I bought Pops old F250 years ago with a 360. It had been sitting for years with gas in the saddle tanks. I drove it a few weeks then switched over to a saddle tank on my way to work one morning. That afternoon, it didn't want to start, was smoking when it did. Ran like crap too.
Pulled the valve covers in the driveway and found a bunch of bent pushrods. Pulled the heads and took to the machine shop. The old gas had caused multiple valves to stick. They had to drive the valves out with a hammer. They had to open the doors to air out the shop from the smell of old stale gas coming from the heads.
May want to have your heads looked at.