If you're sticking with the 126mm axle spacing, then using the narrower 7sp cassette body will keep your wheel dish down, which is good from a strength perspective. The STX-RC freehub body you linked to will fit on your hub, and will also allow use of 11-tooth cogs if you so desire.

The usual fly in the ointment is the fit between the driveside cone/seal and the dust shield. HG hubs need the dust shield recessed, to allow the lockring to thread into the inner wall of the freehub body, so you can't just transfer the dust shield from your old non-HG body. If you could find a whole STX-series 7sp rear hub, then you could harvest the freehub body and the driveside cone together, although some adaptation of the hub spacers would probably still be necessary.