st170 head is by far the best flowing head in terms of cfm , has 1mm oversized valves as standard and has redesigned shaped ports to aid flow. You can make this head flow seriously more cfm with the correct work and it will bolt straight onto the blacktop bottom end , it will also bolt onto the silvertop bottom end as well but with mods to the block etc to mount the idler pulley. Also remember there's the choice to fit a 1 mm oversized valves so 2 mm over a standard blacktop or silver top.
The only problem that I've heard with the st170 head is the twist far more easily than normal zetec heads.

AL developments st170 look very nice as do area six motorsports , I know ian has really gone to town on his heads now and things have moved on a long way , plus he knows everything you need to know about zetec turbos

cheers paul