Remember: you can only add 13,000 lbs. to rear for a total of 34K. Unless you have a heavy front end with a 14,000 rated axle, you will only plate 46,000 GVW maximum.
You also need to realistically estimate how many time you will get those heavy loads, and are you paid accordingly? For example, I have talked to a TSES driver, who will hesitate to use his because the rate is only $1.35 plus FSC for loads over 14,000 lbs. (I would still take those: that is at least $1.50 /mile) If install the axle below, I will be able to plate 24,000 lbs in 22' feet of space. Almost 25K, if no more than 20'.