Isn't most versions of the CF6-80 engine rated at under 56,000 lb. of thrust? I believe the engine they're using on the C-5M upgrade program is the same F103-GE-100 engines used on the E-4B command post and VC-25A Presidential transport, essentially a military-certified version of CF6-50 used on the 747-200B and DC-10-30 models.
The CF6-80 used on the 747-400 is rated at 63,300 pounds. The CF6-80 engines used on the C-5M will be a derated version of the more modern and efficient model used on the 747-400 and the VC-25 (Airforce One). The VC-25 is nominally a 747-200, but has much of the 747-400's system improvements. It has the more powerful and efficent engines of the 747-400 while also having the twin seat 747-400 glass cockpit. With a light fuel and cargo load it can take off from much shorter runways than most 747's.