Are you going RWD or 4X4( )?

If you're going RWD, use a cossie T5 box, they're stronger.
As for the tunnel go and buy a shitty old scrap sierra and cut the floor out of it, that'll make life a lot easier than fabbing it all from scratch.

If you're not too keen on doing the fab work yourself, speak to Andrew Gallacher, he's doing the biz on the Focus me and a mate are building