I've researched converting my '93 D250 to an I beam front axle for a couple years (other projects keep popping up). A front axle out of a 60's
Dodge D200 crewcab will work but hard to find. Its rated at 3500 lbs. I weighed my truck the front end weighs 3100 lbs with driver & full tank of fuel. I figure the I beam axle & leaf springs will be alot lighter than the 2wd crossmember & suspension. There is a disc brake kit available. The ride height of the truck should be about the same height as it is now.
Old Class A motor homes built on d400 chassis axle could work.
An I beam axle out of a P30 step van with 8 lug wheels may work I haven't been able to measure one.