I've been threatening to do that for years, around my Ford fanatic buddies. I am no fan of twin I-beams, though my '66 Merc pickup works all right. The local machinist sez he'd do the welding if I want to try it. It would go under my '62 Unibody to get disc brakes on it. The only problem is that the twin axles aren't straight across the chassis, one angles forward, the other aft. Don't know what that would do to the geometry if they were straightened out.
As far as the welding part goes, the newer Fords have welded-up steel axles from the factory, no problems.