Replacement FH bodies do not include the parts to make the old axle correctly sealed when going from Uniglide 6 speed to combi UG/HG or straight HG 7 speed.

And you cannot use the old UG axle seal because it interferes with the new Hyperglide lock ring. I have done this conversion and know what I’m talking about here. The gap is pretty small on the Shimano 105 six speed to 7 speed conversion I did. So I did not worry too much about a less than perfect seal. I ran it with a Wheels Manufacturing 137mm chromoly axle and the original cones, spacers and lockrings. Slight re-dishing of the rear wheel was needed to center the rim since the 7 speed freehub body is wider than the 6 speed one.