No need to swap tunnels, just take a small section out where the transfer chain cover is on the 4x4 box.

If you stick with the original tunnel then use the alloy gearbox crossmember from a mark 1 2.8 4x4 sierra the one fitted with the t9 box - it fits directly to the mt75 4x4 box from the cosworth and mates up to the original runners fitted in the tunnel, lining up the box perfectly.

meant to say cut the notch of alloy sticking out on the side of the transfer chain cover in the above pic as that helps with clearance as well