If you're patient, you might be able to get it to free up with some penetrating oil. Soak it real good and carefully try to work it back and forth. Sometimes they will free up and sometimes not....
I had to split the manifolds on a GMC 228 and remove the valve and shaft by grinding off the spot welds. I used a new 5/16" shaft with brass bushings, then re-welded the valve back on the shaft and re-assembled the manifolds. On my 235, some penetrating oil did the trick.
Do you have the spring? It's heat sensitive and controls the opening and closing of the heat control valve. Let us know how it goes, good luck.
