dual wheels to single wheels
The rotors/hub assemblies from a D-250 will fit right in place on the D-350.
As for the studs,the holes in the hubs are the same size(9/16). The difference is in the size of the stud at the threaded part(1/2 and 9/16).
The rear axle is wider on the dual wheel axles,but the wheels will fit on the axle.