I'm not sure how much volume your customer expects to do, but it may likely be much more cost effective to modify the container than to try to build a forklift like the one that you describe. The load could be dragged out if on the right pallet or secondary type live floor. A container where the four sided top lifted off of the base would be very nice.

You need to provide more information:
1. Weight, width, thickness of load.
2. Is the steel secured on a pallet, skid or what?
3. How high are the loads stacked in the container?
4. How much "lift off" height will be required to totally clear the load beneath the one being lifted. No doubt the "unsupported" end of the 20' steel plate will flex downward when lifted off & the very long forks will flex a significant amount as the load center will be 1' to 2' from the fork tips.
5. Once you get the answers to what size truck you'll need from the lift truck manufacturer & fork supplier (no doubt a special size fork section - width & thickness - will be required) - you'll also need to verify that the floor loading capacity of the container is not exceeded (there are ISO container standards for this - do a Google search & you can find). There will be one rating for a steel floor & another for wood floor.
6. Most trucker's masts have be developed for the 4-6 k lb. cushion tire units - it is very doubtful the same lift or MFH of 78"- 80" with a lowered height of 59"- 60" ratio can be applied to a larger capacity units.

In a few applications where I have seen long loads light this being received in a container - the container was a soft top (canvas covered) & and a overhead crane was used to unload or a large capacity lift with a fork mounted boom - the forklift worked from the side of the container & it was in a well (about 54" below the forklift operating surface).