Hitting the front seats is okay....in fact, "bracing" is recommended from
Britax--as long as it isn't forced. Also, how old is your child? If she has good head control and is NOT a brand new newborn, you don't have to have that full 45 degree angle. You can have it between 30-45 degrees. As long as her/his head isn't flopping forward when they are sleeping, then you have the right angle.
You can always head to a store that carries
Britax and try it out in your vehicle!