You should be fine with a 5.4 SD, as long as the rear end ratio is 4.30 or 4.10 at the least. I don't think it came with a 3.73. With a Superchip, intake and exhaust, you should have plenty of power. Best place to find one would be at a fleet manager where someone did a trade in on a new one.
2002 F-350 SRW Lariat,4X4, PSD with too many goodies to list. 182k and a very strong 7.3. SOLD
2008 F-250, SC Lariat, 4X4,Shortbed, Pueblo Gold/black, V-10, 4.10, auto, Delivered on 9/7/07, great looking rig. South Texas bumpers front/rear, Air Raid Intake System, Diablosport Predator, Magnaflow, Husky's, JJ's, Ford alarm/remote start, Marathon seat covers, Sirius, PIAA, Access cover, Weatherguard tool box, Rhino liner, Michelins, Duraflap mud flaps, Escort 9500i, MagHytec rear dif cover.
2007 Lincoln LT (wifer's grocery getter),Superchip,Magnaflow duels, K&N dropin, Leer Browning Special
1996 Ford Bronco EB; almost stock
2007 Kubota 900 RTV diesel with goodies.