Well you certainly don't want to use any non-meat heatsink. What are you putting on and how much are you putting on? What size WSM do you have?
You want to be at 350 for a few hours and then back to 300 for how long?
By turning the door upside down and propping it open on the bottom, you'll get some high temps. Then you'll have to fiddle with the vents to back it down to 300.