Hi
Copies should also be fiberglass, if its plastic then you are in trouble!!! I have the original skirts and spats on mine as i wanted to see if they would fit perfectly (due to the replicas needing mods to fit) and even these were a pain to fit and the retail is 3-4 times as much!
Sounds like this bodyshop have not done this before mate, if i was you i would take it somewhere that has as if they are fitting a bumper without even offering it up then they sound like they dont know what they are doing as this should always be done, replica, stock, original etc it doesnt matter.
If they start cutting heavily into it it could loose its strengh and integrity and cause it to be really weak. There are some really dodgy kits on Ebay at the minute, im trying to push mine on there as they are a lot better than most on there!!!
Give Dude a call he is always willing to offer an honest viewpoint.....