You cannot use that flux core welder to weld the tubes to the housing since the housing is cast iron. That will need to be done with a TIG welder and a certain type of rod. Cant remember which rod they mentioned in a video I watched. I do know flux core and mig solid core wire machines wont do it. All the factory did was press the tube in and plug weld the tube in a handful of locations.
You dont need to pre heat the tube, if the machines turned up for the heat all the way, then slow down the wire feed, and work the puddle slower.