The gasket is necessary since the body lining is sitting on a metal shelf. It will seal in the heat and keep the shelf from getting hot.
I purchased the ceramic rope and mortar from Wesco Refractories
I put a bead of mortar and placed the rope on top of it then smoothed the sides.
I lowered the furnace down so to check alignment and to seat the gasket. When the mortar dries it will be in the correct position. The gasket seals very well. I noticed no hotspots coming though.