Hi mercmountaineer:
I've considered the same combination on my Explorer. As others have said, Lightning wheels won't fit on your Mountaineer. What you would need are wheel adapters that bolt up to your 5x4.5 hubs, and have a 5x5.5 bolt pattern on them. A set of good-quality hubcentric adapters will cost nearly as much as a set of wheels, and will push the wheels out at least 1.25 inches. I suspect the tires would rub (at least in the front), but I haven't taken any measurements. I gave up at that point.
As far as I know, no one makes a Lightning wheel replica in a 5x4.5 bolt pattern. I wish they did, because I really like the '99 style a lot.
Another possibility, if you're determined, is to swap out rear axles from an F-150 or Expedition into your rear axle (they may need to be shortened). I'm not sure about the front. I doubt Expedition hubs will fit, but maybe they could be modified, or custom flanges made by a machine shop.
Ultimately, it would be a lot easier to find another wheel that fits. Let us know what you end up with.
Regards,
John.