The 450 is a LOT cleaner than the FLD120 aerodynamically. I use the 450 (or could do 550, but the reason I didn't was that the only difference is the rear axle and springs - and I wanted to be able to change ratios and it is on air anyhow) is because at the time the SD was the ONLY 4x4 in it's weight range on 19.5s, and that is the largest wheel that would fit under my trailer. Even now, the frame on the GM 4500/5500 is too high (in either 4x2 OR 4x4 configuration). The Iveco is probably the benchmark: it gets about 13 mpg towing the same trailer, so I doubt I could do any better with any engine in the Ford. I have had about those results with my Mitsu 6D14T conversions of F350s as well. If I were to do anything at all with this thing, I would probably go with a Mitsu 6D16T and a 9 speed Fuller and divorced xfr case - but the weight would be quite an issue. Might as well just build a real truck instead of trying to turn a sow's ear into a silk purse.
The Ford has used brakes (pads & seized caliper - even WITH the S-B installed rock guard), ball joints (there is NO added load on the front axle, 5th wheel right over rear), various engine parts, injectors, several clutches (hub failures), two gearboxes (pilot brg failures - that cost the whole gearbox) three rear gearsets (location of vent - putting water in the diff), one rear diff and both rear axles (FoMoCo incorrect torque bias on TrueTrac) - all in a measly 140k miles. The HVAC and parking brake have NEVER worked. The gearbox ratio selection is useless (requiring development of Aux OD splitter with the right ratio). On top of that, add thousands of dollars and hundreds of hours to develop and test fuel system mods to make it actually run at all. What can I say? It is not reliable. In fact, it is the least reliable vehicle I have encountered in 40 years as a mechanic, service manager, dealer, operator, etc.