Depends on what torque your putting through it really. An IB5 from a Focus 1.8 will go straight on and the flywheel/clutch/driveshafts shouldn't be too difficult to sort out. It can be strengthen up a bit internally.

You could also look at the ST170 6 speed gearbox (also used in the BMW Mini Cooper S). As its a 2 layshaft layout its about the same length as the IB5, but a lot stronger. Unfortunately it uses driveshafts which aren't similar to any other Ford and also different input shaft dimensions, and a heavy DMF.

Martin