I've never made one of these, but my guess would be several 9x13 cut in half and stacked on top of each other, then cut into the curved shape at the top. That's how I'd go about it anyway. And don't forget to use some dowels in a cake that tall! What website did you find the pic on?
Forgot to add ice and cover in fondant, although that's pretty obvious! 
see diagram:
cover in BC, then in fondant -- I'd do three pieces...one each front and back and then the strip for the sides. mush edges together...ok if not prefect, will give effect of carved stone edge better
also don't worry if fondant cracks some...that would be like a crack in the stone (for once, the problems are actually the solution)
paint up in greys w/ black for the cracks.
add lettering to front (might be able to do this while front piece is still off cake and on counter)
I know this is late in the game, but using the above method (thank you thank you!) how is transporting?
AHere is instructions on a tombstone cake I used these instructions for the zombie head but didn't try the headstone but hope the instructions help you. http://m.instructables.com/id/How-to-make-a-sculpted-zombie-cake/step3/Building-the-tombstone-support-and-cake-board/
Thank you for the responses! I wasn't sure if I needed a back support or not. I am not planning on mine leaning back in the instructable. Just pondering what to do :) Thanks again!
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