"Paul Kopit" wrote:

You would need to put on a hub body, which costs $25-35.
You can get by with the existing axel if you can add 2 mm
of spacers to the non drive side of the hub and center the
axel between the axel nuts.

Since an 8/9 speed freehub body is wider than a 7-speed one, it's usually
necessary to remove some spacers to compensate for the extra width.

You'll need to redish the wheel slightly.

That hub will go into 126 mm spaced rear dropouts.

In 1991, the DX hub came in both 130mm and 135mm versions. The 135mm model
is the simpler conversion for two reasons. First, it has a broad spacer on
the left that can be removed to compensate for the wider freehub body.
Second, these hubs had slightly wider flange spacing than more modern hubs:
a 130mm 1991 DX hub with an 8-speed freehub body transplant ends up with a
very large left-right spoke tension disparity.

If the dropouts are already 130 mm, you can skip the fooling
with spacers.

If the dropouts are already 130mm, it probably makes sense to spread them
to 135 and respace the hub to suit.

I'd recommend that you get 9sp barend shifters and just 8 of 9 of the
9sp cassette cogs on the 7sp hub body.

That seems like the best plan to me.

James Thomson