Given the fact that even the short 6. 2L has its last 2 cylinders under the firewall and in the doghouse, I can't imagine what would have to be done to make a 5. 9L fit - do you really want to extend the doghouse another foot? The reclining radiator takes up nearly half the engine bay, so moving the engine forward won't help unless you can somehow find a way to get equal cooling with a small radiator mounted in the traditional location.
If you're planning on using the Hummer for off-roading, the 5. 9L won't let you do anything you can't do with the 6. 2L. I've seen more than a few 4-litter armored humvee ambulances stripped of their medical equipment to make room for installation of radios, fire direction computer, map board, etc, to make an artillery battery or platoon fire direction center/command post and the 6. 2L moved them along just fine. Add the camouflage net sitting on the roof, sandbag-lined floor, 3/4T trailer carrying personal gear, 2 4. 2kW generators, fuel cans, water, rations, etc, ad nauseum, and you're probably talking close to 12,000lb being moved cross-country by the lowly 6. 2L - and they were just as mobile as the much larger and much more breakdown-prone M577 tracked command post. I think a 4BT would be a much more practical repower for a Hummer, and you could find 4BTs backed with TH400s in many bread trucks.