only proble is that fibreglass has a different flex rate to plastic and its fitted on a plastic bumper and to another peice of plastic, it will crack pretty quickly, then you will have spent alot of money on doing it wrong,
 
you could cut slits in the lcrs rear bumper and put fins of abs sheet plastic through them, weld them up neatly either side then sand it all down nicely and get it sprayed up.