I did a similar project 12 years ago, when the frame on our "purchased new in 1979" Dodge D-100 rusted so bad it broke. Using just the drivetrain/chassis of our deteriorating 1974 Dodge Vagabond motorhome(with only 35,000 miles), I created a heavy-duty dually with a 7,500 lb rear axle and I-bean 3,500lb front axle.
Found a more recent pic with black wheels and 10" gap filled with spare tire and toolbox (did not alter motorhome wheelbase).