if you go b18b (jdm) keep your old motor you'll need your driver side bracket and intake manifold if you wish for everything to hook up....cuz it comes with an obd0 mani......I can tell you that much.
As far as B20 goes.....get a B20Z.....I think you'll aslo need to change the manifold as well.....
I agree ls boosted is a very nice respectable motor and has been proven to have some nice gains......I'd pick the b18 over the B20.....I mean unless you want all motor....then B20 would be better.