To avoid the 6. 0L problems in 2004, my fire department (per me) got an F550 4x4 V10 5:38 Locking Diff 84" Cab-to-Axle Automatic. 315HP and 425Ft-Lbs.
This truck is very capable... more than capable. Gas MPG is around 12-14 unloaded in most of these under good conditions. Many utility buckets for the power utility use 550's with V10's. They like them as much as the PSD versions. They will go 350K miles with good care... even more. They are HD engines and definately a good choice. The utilities run them 8 hours solid a day and are not easy on them.
Plugs are EASY to change... just takes an hour, socket, extension... a little dusting of neverseize on the threads and 14FT-LBS. Folks who say they are hard are just not used to where they are in the heads. If they are not between the exhaust manifolds... folks say they are hard.
The blowout of plug issue is 99-02. 03's and 04's had more threads in the aluminum heads. Early ones can be kept from this if you torque properly and check torque every other oil change. 2005 and up have 3V heads and the plug issue is also not a problem.
If one does blowout... don't buy new heads. An afternoon and a TimeSert kit made specially for these Triton motors is easy and a permanent fix. Some dealerships use them as well. It works.
I'm totally satisfied with our choice. Runs all day and it WILL get up and move. Just don't be afraid to take it up to its redline. It screams... but thats where its power is. It will run high rpm all day. Although with OD... it will just purr down the highway even with 5:38's. Ours runs 85mph on the highway with way more left. Its electronically governed at 85 which is fine for our needs. I don't need guys running 100.
Easy to maintain, works hard, fuel MPG is same as todays new PSD's, and it was standard engine in the F550... ie: saved us $6000.
All just my opinion... but I'd get a used F450 cab chassis. They are easily available and you'll love the 14,000lb GVW over the 10 or 12,500 of the 350. Our 550 maxes out at near 18. Heck... this thing would haul our old complete F350 2WD squad... tools and all.
My next door neighbor has a 2000 F350 4x4 flatbed and he pulls a HD tandem axle dump trailer all the time with busted up concrete. He used to pull it with a 93 Ram 350 Cummins. His claim is that it handles and pulls much better than his ram. Just at a higher RPM... which is just what the V10 likes. Of course his Ram was about half the horsepower and had equal torque of the V10.