In all honesty unless your preprard to seriusly modify things i would personally stick with a bc box from gearbox man upto 320bhp.

It is possible to fit the 6 speed box as i have just done on this on my zetec turbo, but its not easy..
The difficulties come with custom drive shafts, custom mounts, custom flywheel and clutch set up. The st170 6 speed box uses a dual mass flywheel and clutch set up this is binned and uprated one is needed.

the other area to consider is the box does nt come with a good diff so a quaife one is needed to be added. The box also runs a hydrulic clutch set up and due to its sixe in the engine bay the standard brake servo is needed to be binned and a bias box installed with a hydrulic clutch pedal.

I would say your looking at circa 2k to install the above not a penny less and thats if all fabrication work is carried out by yourself.

If you need any pics let me know il PM you some