After a bit of reading on xtremesystem's forums it looks like you can keep the pump from cracking at lower temps by insulting it.
http://www.xtremesystems.org/forums/...ad.php?t=38810
Not sure if an Iwaki would be a good idea then. They insulated the entire pump. Iwaki's get hot. And you can't insulate the pump head from the pump housing which is metal and gets hot from the motor.
Pump head at below -40c attached to a metal frame that gets up to 40c+ would probably much too large of a temperature differential.
Lots of people with chillers use Danner mag drive pumps. That insulated one in that thread is a Mag5
edit, same thread they talk about submersing a mag pump so you dont have to insulate it. Will acetone eat a rubber seal? I think it will =\