42L, BMI 40. Too Obese for Breast Reduction?

Only if you can not receive a medical clearance than you can not have a reduction surgery. My choice might be amputative reduction mammalpolasty. 

Thank you for the question.

At some point you will be an excellent candidate for breast reduction surgery;  it will be in your best interest to reach a long-term stable weight prior to proceeding with the surgery. This recommendation is based on safety of the procedure as well as predictability/long-term outcome of the surgery. Insurance company coverage likelihood will also be improved.

Best wishes.

Based on the information that you have provided it is unclear on whether or not you would be a good candidate to undergo a breast reduction.  There are many factors that one must look at in determining whether you are a good candidate.  I recommend you schedule a consultation with a board certified plastic surgeon so that they can give you a physical examination and determine the best route for you and your body type.