As above, Mine being a base model 3dr sierra has the narrow type tunnel, How easy/difficult is the swap to the wider tunnel from a cosworth?

 

its will be easier to mod ur tunnel ,to take the box or case get the grinder out and cut the hole thing out

 

T5 gearbox fits the base 3 door without any mods.

4x4 gearbox will need some chopping.

 

as mark said only the 4x4s need modding. mine is modded for a 4x4

 

Have a look at my restoration thread ( not sure how to put link up) I've done this to my base spec replica .this was to fit the 4x4 box though.
Easy enough to do , just measure twice cut once

It's in restorations under: 4x4 3 door replica !

 

keep it rwd, more fun and no mods needed.

4x4 is for gays

 

Ha ha funny . But you can make use of all the power you have.
Must admit though would be good to have it tail happy

 

At least with rwd you can put your foot down properly in third without worrying about the transmission exiting from under the car