I don't think there can ever be a definitive answer when building one-off vehicles. Much depends on intended use. Horsebox construction in the UK at least usually sees the habitation box (overcab luton included) flared into the vehicle cab and they don't seem to tear themselves apart in highway / getting-on-and-off-fields use. Builders seem to pay little attention to the concept of chassis twist. Maybe the extra heavy duty floors they use to support the weight of horses helps keep the whole structure rigid.
I suspect, though, that if they ever approached the limits of suspension in a cross axled manner then things could turn bad pretty quickly.