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.
The F103-GE-100 used on the E-4 and VC-25A are rated at around 53,000 lb. thrust, and I believe that's more or less the same engine that will be used on the C-5M. Gawd, it will be so strange to hear a C-5 fly over sounding more like a 747, given that you won't hear the very distinct loud whine of the old TF39 engines.