Can I cut fittings to get flange to fit?

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?

a few more pics could help
i would flip the street 90s and go with a hub flange
if the back wall is at the top of the pic
then maybe a 45 with a 45 flange
hard to say

No, you can not cut the fitting. reverse them so that the street is pointing up and use a female flange.

There could be several ways to do it without cutting off anything. A "spigot" flange, a 4x3 spigot closet bend (which is the way I ALWAYS do it), or a street elbow and a hub flange are three possibilities.