Spring stiffness needs to be in proportion to weight they are carrying. Heavier springs just for the sake of stiffer suspension simply leads to a rougher ride.
And for what it's worth, yes, Ford makes the stripped chassis in both E-350 and E-450 dual rear wheel models. A few years ago they even made a lighter-duty version of the stripped chassis with single rear wheels. Unfortunately, the SRW version is no longer included in the comprehensive Ford specifications.
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