Overheating new 360 help

Since this overheats after some period of time, that seem to indicate low coolant flow or low airflow, or a combustion leak into the water jacket. I.e., the cooling system just is not pulling enough heat out. IMO, chasing a few degrees of advance above 38 degrees at 2500 RPM is fruitless. But see note 8.

1. Is this a standard mechanical fan? How many blades?
2. Is there a shroud installed? If not, then this could be the issue as it is needed at idle to make the air go through the rad.
3. What are the rad dimensions, including depth?
4. Pull water pump and make sure the impeller is not loose.
5. Get an old t'stat and gut out the moving parts and use that for another test. No, don't run with nothing in that spot; you need a flow restriction there to keep pressure up in the heads. Watch the flow in the top of the rad while doing this.
6. Watch for the lower hose to be collapsing on itself; make sure there is a spring in there.
7. Get/borrow a cooling system pressure tester to look for elevated pressures and for pressure pulsations. This would indicate a block crack or a head crack or a poor head gasket seal.
8. Find true #1 TDC and check the timing mark on the damper to be accurate. If it is off, then you could think the engine is at 38 degrees advance but actually be a lot less.

And with the 2 freeze/core plugs being pushed out.... that is bad news. Keep an eye on the oil for water.

And I would not consider the slow draindown in the rad while filling to be abnormal. A very small hole drilled in the flat flange of the t'stat will help this.