I have a feeling he could be reffering to a fuel cell. I thought about cutting the wheelwell out and smacking in a fuel cell when I had my 88 gt to gain clearance in my tire length after moving **** inward. But I said naaa. The stock tank just looks good with the fatties I have now.
if you decide to cut into that area, cut just enough around the wheelwell so you will have room for the relocation of your battery and maybe other stuff. I would also underbar it by welding some steel plates so the sheetmetal won't warp out as sooner or later heat will take over and cause it to expand and then bend.