The 6000 series front axle was available as a heavy-duty front axle RPO on the 4000 series, so it should be a bolt-in affair (as a complete unit with the corresponding spindles and hubs). The 5000 series axle was a slightly heavier I-beam than the 6000, and the track might be a little wider, but I think the spindles and hubs were the same as the 6000, so it would probably work too.
