Good job on the build. Been following it for a while now. Thought I would pop in an share my 2 cents knowledge.
Wood, will burn eventually, with that high heat.
Try to use stainless if you can. Will take the heat and the abuse from the road.
If you want granite, use the 12" tiles. Easy to cut, not too heavy and easy to install. You can do an under frame with angle iron. Try to use 2" angles. Then put hardy back at the bottom, and do a scratch and brown over it with diamond lath.
Once dry lay your granite tiles, very tight, no grout in between.
Hope that helps.