if your car is 96-98 and you want to convert to 99-00, technically you don't need the rad support. from what i can tell, there isn't much difference between them. it might take a little custom fitment to get it to work, but i wouldn't think its it would be that involved. unless you want everything to bolt up absolutely perfect, i don't think it is worth cutting off your rad support and welding a 99-00 one on.
if you have both the cars side by side (your bros 98 and the 99 si) try swapping all the parts and see how they fit. then you tell how different they are.