There are 2 versions of the MK3 shell & MK3.5
MK3's - 1 for the lesser models & 1 for the XR2i & Turbo.

The mk3.5 are similar however as I think after looking on Microcat that there are 2 different shells as well.
The base 1100 & 1300 will also accept the CVH lump as a straight fit but is the shell strong enough to handle the power from say a 200 bhp tuned turbo lump?
The other shell is for all Zetec engined cars so the Si is the same shell as the RS1800 which can deffinately handle 130 bhp so my advice would be to go for this one.

hope this helps.