Replace thermostat with the correct one.
The 6.2L takes the “hockey puck” style.
Other standard style will work, but is wrong for this engine.
This is the correct one.
https://www.rockauto.com/en/catalog/chevrolet,1984,k30+pickup,6.2l+379cid+v8+diesel,1053021,cooling+system,thermostat,2200