That was an understandable conclusion if this was your intent in 1985.My reasoning is if I start at the top (F-600), I'll have all options/ability to tow vs. starting with an F-350 and later needing the ability to tow what an F-600 can tow.
Light trucks these days have more tow capacity than a Class D license.
I question the very idea of building a trailer/home/4x4 hauler that weighs enough that you need an F650 to tow it.