hmm well can 3.90s fit in a 3 series carrier? they arent' that far from the 4.10s

 

I put 4.10's in my car and they made the world of difference.
I don't think you need a new carrier I put my 4.10's in myself and it had 3.42's with the 3 series carrier. I think 4.10's are the limmit no bigger than that. You will be ok with just gears and installation kit. No carrier needed