ive been led to believe that you can run without the extra gearbox mount but personally i wouldnt recommend it as its a bit yeehaa when there is no need.
you will need to aquire some rs2k hubs aswell (so sorry for not saying this before i comlpeatly forgot. if your gagging to get running you can just run the 2k on the zetec box but you might kill it after a while. im using a diesel box from a mk4 and it hasnt broke yet......it has only been a month or so though that ive been up and running though, but then im in the motor trade and when it does go bang it will only take me a morning to put another one in and they arent hard to come by.
by bigger box i have no idea what you mean im afriad though.