The gearing on both will make the biggest difference. It won't be a powerhouse, but I expect the 5.4 will be fine in Tow-haul mode. Worst case, you can improve the power by relatively simple mods like K&N air filter and more free flowing catalytic converters, mufflers, and exhaust pipes.
BTW, I towed a 6000 lb trailer over the Cascades for a while with a '97 5.4 Expedition. My guess is the overall weight should be about the same although the 2008 5.4 should be more powerful by quite a bit.
'10 Ford F-450, 6.4, 4.30, 4x4, 14,500 GVWR, '06 Host Rainer 950 DS, Torklift Talon tiedowns, Glow Steps, and Fastguns. Bilstein 4600s, Firestone Bags, Toyo M655 Gs, Curt front hitch, Energy Suspension bump stops.
NRA Life Member, CCA Life Member