If your going cams and ITB I'd run a 3" for sure!
I'll be going up to a 2.75" or 3" when I put stage 1 cams in my B18C and get it mapped again.
Just get a custom 3" with a spoon n1 style back box
FWIW I fitted a 3" system to my old DC2 and it did not like it... I'd stick with 2.5" on a B series for sure.