It just so happens that I have a 88 frame and a newer swing arm with linkage that I could try it on.
I believe that you can use the 88 frame but you would need to use all the newer suspension from the 98, not just a shock.
IE swing arm, linkage, rear brake caliper and mount, shock, and possibly sub frame.
The 88 k5 shock has a single mounting post whereas the 89-04 k5 used the H shaped clevis.
More tomorrow.