Help with load bearing cantilever design

I am building an outdoor fireplace and I want to float the hearth using either flat iron stock or angle iron that will be mortared into the joints of the cement block. I would like the hearth to support up to 1000 pounds (which includes it's own weight). A quick search on the weight of granite suggest that I am looking at about 450 pounds for the hearth. This is based on 2" thick granite, 15ft2 at 30 pounds/ft2. The granite slab would be about 2" thick x 8' wide x 20" deep. The idea is that the angle iron or bar stock would cantilever out 18" to support the 20" wide slab of granite. A couple inches short to help hide it a bit. I would prefer to use just flat bar stock, so that it is hidden the most, but I will tolerate angle iron if it is stronger/needed. If I use angle iron I have only 5 five joints that I can place it in. If I use bar stock I can use as many as needed along the entire 8' length of the hearth. The Joints of the cement block about 3/8", so the metal can not really be any thicker than that.

If it matters for any calculations, the length of the iron that is mortared into the fireplace can be a max of 31 inches.

How much 3/8" thick angle of flat iron will I need to support my hearth?

Thanks for your help.

Couldn't seem to find the "Edit" button for my post, but I wanted to add that we can assume a 2" leg on the 3/8" angle Iron.