Yes it is hard, poss a little easier with a zetec than a CVH like i am doing but it isnt easy by any means!

So far i have had to do the following to fit a focus RS MTX75 (rs2k are slightly easier in some ways):-

Cut N/S chassis leg alot to box round the larger gearbox

Cut a large hole in the bulk head to accept a hydrolic pedal box

Have a focus zetec alloy flywheel holes slieved to fit the smaller crank bolts that the CH take - thought i may as well go for a fidanza lightweight one rather than having a std focus one machined down a little!

Use a focus RS clutch - i had a custom 6 paddle helix item made to reduce the heavyness in the pedal slightly and make the car easier to drive in trafic.

Have custom mounts fabricated so the box will mount to the car - i am goint to use the focus top mount and then have two link mounts to prevent any swing in the engine and box

Have a focus 3 piece shaft shortened to fit into the fiesta

As i am converting an early mk3 to run an MTX75 i had to swap the front hubs, suspension and lower arms to a mk3.5 setup so that the focus shafts fit into the hubs

Also i have got to basically make a one off centre console to accommodate the cable linkage off the focus with lift up reverse (the focus one will not fit by any means so a mk3 one needs alot of fiberglassing!)

Thats all i can think of off hand but is not by any means a complete list.