Is five miles the extent that you have driven it? I am wondering if you still have air trapped inside. If you start the engine and the temperature goes beyond the thermostat setting, check the upper rad hose. If the rad hose is cool, the thermostat isn't opening because of an air pocket. What happens next is a void forms in the block and the expansion pushes the coolant up into the rad and out the cap. I have been through this with my 216. What I do when I initially fill it up is I wait until the fore mentioned happens and then I shut off the engine. Then I remove the heater hose on the thermostat housing, hot air belches out and as soon as coolant appears, I Push it back on. After a few minutes the thermostat opens and I can run the engine normally. If it were me, I would start off with a 160 thermostat until everything is resolved.
