when i said 'new' motor, i meant a 'new' junkyard motor. no one buys a 0 miles motor unless they are crazy.
a lot of the issues come when the coolant isn't flushed out of the motor properly. dump a quart down the valley and wash the coolant down into the pan, do an oil change and just run it idling for a bit, do another oil change, etc.
i find that most of the time these motors die because of a mistake made when working on them. otherwise they really are tanks.