Installing 4.8 Tips on 3.0

[FeelGoodUnInc]

I bought the oem 4.8 tips from Center BMW (Van Nuys)..took it to a trusted local shop (California Muffler) and they told me the install would not work and/or the install would be too much of a risk..even if they did try the install they said it would be extremely difficult to keep the tips looking straight...when I asked a couple guys from Center BMW and Crevier they said the install would be no problem so I'm confused as to whether to return these tips and go for the whole replacement system or try another shop to install these although I trust California Muffler and what they say but at the same time I'm hoping their wrong....well any suggestions/tips would greatly be appreciate, thanks for your time!