I'm about to make the swap of a '93 Mark VIII engine into my '98 Expedition 4X4. The research I've done makes me believe it should not be too much of a problem. The major concerns are the ECM and the exhaust, especially the driver side exhaust, the Mark VIII has the Cat. welded directly to the manifold and there may be some interference with other chassis or drivetrain components. If there is I hope to get around this problem by using the left side exhaust manifold from a Mustang Mach 1 that is the same as a Cobra.
The ECM is a guessing game but I believe the Expedition unit will work properly as the same swap has been done in many V8 Mustangs and T-Birds. The pin count and arrangement of the Expedition are the same as the '96 Mark VIII. The only problem I can see so far is that several of the sensors may have the connecting wires cut and spliced as well as using the basic engine wire harness from a '96 Mark VIII as the connector pins changed from a large diameter on the OBD I system to a much smaller diameter in the OBD II system. I also intend to use an electric fan for the cooling system instead of that big clunky OEM fan and that way there may be enough room for a turbocharger.
After the bolt in procedure the Expedition will go to the muffler shop for pipe fabrication.
Another feature of the '96 Mark VIII engine is that it uses the same coil packs as that used on my Expedition
I will post the results with pics when the truck is driveable