Just a quick thought....
On most of he older cars, GM has updated chips. I had the same problem on my 91, with an aftermarket ($375) chip that detonated a lot. Though I had the origonal stock, a member tipped me off that there was an updated GM chip. I got it, and it cost $47, and I have loved it. I have tried two aftermarket (DRM and performance from MAD) and the origonal and the updated stock. By far the updated stock is the most frivable, and actually the fastedst, because it does not have to retard the timing as a result of pre ignition.
Just my 2 cents. Try and see if an updated chip is available for your car before you get too deep in a hunt for something else. It's cheaper, and you may be surprised at the outcome.