If it doesn't block the entire shelf, you should be OK if it's just up against the corners. It'll shrink pretty quick, so you should be fine. Whole is always better, if it'll fit. I'd maybe set at 240. One thing you don't want to do is cook brisket too fast. This is such a tough cut, it really needs the low and slow approach - no matter how long it takes. Just plan for 2 hours per pound, and hope it's done sooner!