408 Magnum stroker running hot.

Remove the lower hose and fill the radiator holding your hand over the lower outlet. Radiator should empty in a gush. I've read the 3 core al is not as good as the 2 core 1.25 tubes. I run an OEM 3 core 22" on my 340 in FL with a 2 speed taures electric fan. You could try a mechanical fan. Thin cylinder walls is also a possibility with a magnum if it has a lot of core shift.