Sara,
First let me say that at 8 months my Marcie was 47 lbs. so it sounds like Summer's weight is just fine. Please remember that Boxers do take a long time to mature. Dempsey took 3 full years to fill out to his current weight of 74 lbs.
Secondly and more importantly please don't be offended by Matt's post. Breeding, while one of the most natural things any animal does, can also be very dangerous. The links that Matt gave you are for your benefit and education. While I agree with Matt when he says
The only really valid reason for breeding is for the betterment of the breed
I also believe that most breeds wouldn't be around if they were only bred for the "betterment of the breed". If that were always the case then there wouldn't be any "pet quality" pups, they would all be "show quality". If you're still interested in breeding, then at the very minimum please make sure that you do wait a
MINIMUM of 2 years and that you do the testing of both the dog and the bitch where Matt's links pointed you.