The zetec has the same standard bolt pattern from what I remember, so you should be able to use a MTX75, or the Getrag 6 speed gearbox from the ST170. The v6 duratecs will have a different bolt pattern as do the newer 4 pot duratecs.

The zetec is no feather weight, but at least with the YB the exhaust will be at the back so the weight of the turbo will behind the front wheels. You'd be better of with 2.3 turbo duratec than the YB IMO as it's a lot lighter than either the zetec or YB.