I have always been an advocate for keeping the air leveling system, because it is often cheaper to repair than converting.
RockAuto lists the same part numbers for both vehicles (88,9
for rear struts, with my understanding being that no C-Bodies came without ELC.
Assuming the spring dimensions are the same, my biggest concern is the weight difference between the 88 and 98. The extra bits of the 98 hang over the rear wheels, so I wouldn't want to jump in to it without knowing the specifics.