I would definitely use one. If you dont have one you can place a couple of flower nails in the center. I have heard from other CC'ers that this has worked. But if you are going to use a heating core, with a cake that big I am not sure if you would need to use 2....but to be on the safe side, I would. Hope this helps 
I have read severl post about using a flower nail instead of a heating core for larger cakes. I just did a 3 tierd wedding cake 12-9-6 and 1/2 a sheet cake using a flower nail and it worked perfectly. I alway baked at a lower temp and watched the cake closely, but the flower nail was super easy and cut down on my bake time! Plus, it leaves only a nail sized hole so, no need to fill in with cake!
You need to grease it and then put it in the pan with the flat side against the bottom of your cake pan, pour your batter and bake. Once cooled, flip the cake over to get it out of the pan and pull out the flower nail. HTH 
Yes I have used the flower nail as well and it works fine. I prefer the core myself, but if I can't find the darn thing, I find something metal to use in its place...
I never use cores or flower nails and haven't for 30 years. Baking strips and a reduced baking temp works just fine for me. I've just never liked the idea of a big hole in the middle of my cake, nor did I want to buy a bunch of flower nails that I'd have to dig out of a cake.
Just never saw a need for them, personally.
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