The infamous Rear Muffler Bypass - by far the most cost-effective modification.

Being that my car was an '85 with an '87 rear clip, the exhaust was already a bit of a hybrid. Just the fact that it's an '85 means that the bolt-on RMB's out there are not for me. Combine that with the '87 muffler tacked on the end of the '85 exhaust, and it meant some welding was in order.
A trip to the exhaust shop & $40 later, I can finally hear the engine, and don't need to convince people there's a 5.0L V8 under the hood. Supposedly, it's worth a few hp, but I've yet to visit the dyno.