I only have 2x6" joists where I'm installing my new bathroom, and the 90 degree elbow that the toilet will drain to is more than 6" tall. Without having some space between the top of the fitting and the top of the joist, the flange that I place above the fitting will ultimately sit above the level of the floor (and thus the toilet will sit off the floor). Can I cut down the cuff of the elbow so it sits lower, and then cut some of the cuff of the flange off to narrow the gap and allow the flange to sit at floor level?