I'm looking at an application to use the T660 coolant heater but need to know if the unit has a built in circulating pump or does it rely on a thermo siphon for coolant circulation?

It is an inline heater cut into a heater hose, no pump. The warm coolant just finds its way to the block for easy cold weather starting.
If you need to plug your sled in, it's probably really cold outside. I would stay home.