If you go here, you can cmopare the specs for the vans you are considering:
The frames are the same between the E250 and E350RB. But E250 springs are softer, and it may
have a lower rated axle. Unfortunately, even Ford's own publications fail to account for all body configurations. Some vans listed with a semi-float rear axle may axle come with a Full-Float (Most EB passenger vans will actually have a 7800# full float axle for example). So.... when shopping it pays to peak underneath to see which axle it really has.