The Stovebolt Site Forums The Shop Area Engine Shop Is the motor frozen, or just stuck? "it ran when I parked it there"

The front inside portion of the valve cover on my 49 216 collects quite a bit of condensation during the colder winter months. It isn't unusual for that white milky telltale sign of condensation to cover the inside of the VC as well as the valve train of cyls 1 & 2. Real similar to what the rust has covered in your pic.
If that truck was run a lot during it's final winter, collected up quite a bit of milky white condensation then parked for years it "may" result in what we're seeing in your pic.

Oddly enough my 216 has no condensation build up in the warmer months.

You may want to pull the rocker shafts and clean them & the rockers up and give those front valves a couple taps with a BMH to make sure they're free and knock off any rust that may have trickled down to the valve and seats.