That's a 2-core aftermarket radiator with no shroud and no engine-driven fan, and one small electric fan; it's no wonder it overheats. When they transplanted the big-block, they didn't bother to upgrade the cooling system to match. Factory-built big-blocks (396) used a thicker 3-core radiator, full shroud, fan clutch, and special fan; set it up with the correct factory parts and it won't overheat.