Nothing. I dropped a 99 in my 95. Straight swap. Use the 94 sensors in the 99...but keep the 99 injectors. Unless you dont have the 99 wire harness...if you do then swap the injector connectors onto the 94 harness. A tune will get you the most bang for yoir buck to help with the larger injectors...they arent bigger by much but it makes a difference.