If you are dead-set on swapping to a cummins, you may be interested in this link:
http://www.ford-trucks.com/forums/664199-excursion-v10-swap-to-5-9-cummins.html
I imagine finding an excursion with a V-10 would be easier and much cheaper to acquire than a diesel excursion, plus people have already done them, so the learning curve wouldn't be as steep.
Keep us posted with your decision. A cummins powered 8 seater sure would be nice. Although, still nothing wrong with a trusty 7. 3. Even the ..... 6. 0..... could be a good choice if you took it straight to a guy like Bill from
www.PowerStrokeHelp.com and had him "bulletproof" the motor for you. I understand it isn't really that much to have done, and THEN they can be strong, reliable motors. I know he does these motors all the time on pickups. He pulls the cab. I wonder how he does it with an Excursion, being there is no "cab", but the whole body...
Interesting nonetheless.
And no, I don't need to hear how the 6. 0 is junk. I think we all know that. . but they are good motors when the few weak links are addressed. Save your flaming...