I believe the Buick 350 has a bypass hose at the water neck which could account for a stuck thermostat and hot upper radiator hose. Some water gets by no matter what. Couple thing I would check, heat gun across radiator, check heat drop from top to bottom, as earlier stated pull t-stat and check with a thermometer on a stove in a pot of water for the actual opening temp, the "cracking" temp should be a few degrees before 195, it should be fully open at 195.