Enjoy the heatwave, Jim!
I think a select grade will be fine, with proper prep work. The grading, from my understanding, considers marbling and maturity, as well as other factors (like texture, coarseness of the grain, and color). Obviously, USDA Prime is the best grade for all of these. Choice is second, and Select is third. There is a Standard grade below these, too. This is the grade that stores will usually rename (because they're allowed to) to things like "Special Reserve" and the like. It means it's Standard grade, but most consumers don't know.
As for Select: I dry-aged a 14 lb ribeye for 21 days, and it was Select. Best steaks I've ever had. So, prep can overcome grade sometimes.
What I mean by proper prep: The brisket, as I'm sure you know, is a fickle beast for sure. But, with good injection, rub, and proper smoking, you can tame the beast. I want to try Walt's brisket brine and injection. He swears by it! I, personally, usually inject with my own concoction and then rub/rest in the fridge overnight. I also don't foil during smoking - just let it ride at 225 until it hits 195. Then wrap and rest. I think this will work fine for a select cut. Just my 2-cents, though!
