newly built 416 getting warm

I would think it's an airflow issue because it's fine when moving. So really, you can either get more air thru it, or get more surface area fo that air to flow past. The factory ones are 2 core. You could have your tanks and brackets put together with a 3 wide core. Also, as said, it should be running a little cooler after the rings finish seating. Also, make sure you changed your break-in oil after the cam breakin. What oil pump was installed?