Speaking of the F600/700...
I think I have pretty much decided that my next welding rig truck will be a '95-'98 Ford L8000, with the 8.3 Cummins ISC. At 26,000 GVWR, with single axle and hydraulic brakes, it can still be driven without a CDL. There are L8000's out there with over 30,000 GVWR and air brakes, but those require a CDL and I don't have a need for that much GVWR anyways.
The L-series trucks appear to have been the replacement for the old F-series 500-800 line. L-series were available in 6000-9000 models. Some had the Ford 7.8 turbo diesel and some had the Cummins 8.3 ISC turbo. The L-series was discontinued by Ford and sold to Sterling/Freightliner and Ford came out with the F650/750 trucks in '99, if I remember the year right.