First, I'm retired and do NOT have any inside information--legal disclaimer. But, speculating, with the C-5M program going ahead and, open press, the A-models won't be re-engined, I suspect the answer, for a civilian freighter option, would be to purchase the required number of "A"s from the Pentagon and piggyback on the Lockheed-Marrieta line. There shouldn't be major differences between the two models, the engineering has been done (two "A"s will be re-engined under the developmental contract) and it should be just the variable cost of production AND the FAA certification costs. That could be expensive, I don't know how far Lockheed got with certifcation back in the '60s, but it would all be out of date, anyway. The C-141 was cert'd, but nobody needs another IL-76.
For what its worth, the C-5 and the AN-124 have similar cargo space, but the C-5 has roller bed provisions, better flooring, pressurized cargo space (fully accessible in-flight) and better loading capability. I have never seen a load that couldn't be loaded in under 8 hours with proper preparation. Even loads that required multiple re-configs and removal of some aircraft equipment to make them fit.
GF