The TF-39 engines in the C-5A puts out only 41,000 pounds of thrust. The TF-39's in the C-5B are a little better with 43,000 pounds. I can't understand why Lockheed didn't put more modern CF6 engines in the C-5B when they originally manufactured them. The availability of spare engines for the C-5's that have not yet been upgraded should improve as more C-5's get converted to C-5M's.
I think it was the original 747-100 that had around 47K of thrust per engine. Maybe the high wing of the C-5 helps with lift and obviates the need for more powerful engines??
For those who want to hear (and see) a C-5 take off.
http://www.flightlevel350.com/viewer.php?id=333