No problem in getting those mastadon briskets on the WSM. Just cut off a chunk of the brisket from the point end such that the main section of the brisket is 18" long. You'll be cutting into the point and not the flat. Rub that hunk of point and set it on the rack next to the rest of the brisket. That hunk will make very tasty "burnt" ends by the next morning if you're doing an overnight cook. That hunk is my favorite part of the brisket anymore.
Save some of the large hunks of hard fat that you take off the fat side and tuck those hunks underneath the ends of the brisket. They will take the brunt of the heat coming up around the edges of the waterpan and protect the meat itself - works great.
Keri C, still smokin' on Tulsa Time
2 well-worn WSMs
2 Cookshack FEC 100 pellet munchers
1 ECB circa 1981
1 Stumper clone, size XL
1 Vermont Casting gas grill (in case of emergency)
1 partially converted stumper clone based on a 1965 Montgomery Wards upright freezer