I've played with swapping 7 and 8-speed freehub bodies on various Shimano hubs. They aren't all cross-compatible. In particular, bodies from "road" and cheaper mountain hubs have less space from the last cog to the surface where the cassette body meets the hub body. On more expensive "mountain" hubs the body is recessed into the hub a bit more, sealing this area a little better.
Good to hear your swap went well.
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Jeff Wills
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