to do the bumper conversion properly, you will need to change the front valence, then the front bumper will bolt directly up. The back requires a bit of work though, as the early model cars (except convertibles) have a higher rear wheel arch. The ends of the plastic bumper will not fit flush with the wheel well, and if you want to do it properly, you will have to modify the ends.
see R3vlimited for detailed instructions:
http://www.r3vlimited.com/Tech/bumper1.htm