The fan should cut in just after half way.
to bleed the system i leave the cap off, and take the small hose ontop of the thermoustat housing off till water spurts out.

Does your thermoustat open?
is your fan switch working?
bridge the fan with a small piece of wire between the 2 connectors on the plug (fan side) when the temp guage gets to around half way and see if it cools it down. that's if the fan doesnt come on.
is the header tank camp making a sufficient seal, as the system will not cool if their is no pressure.

Is their anti freeze in the system? i think a 60/40 mix of anti freeze is right.
is your rad hot all over, bottom/top? might need a flush? did your old headgasket go or did you just replace it when fitting the stg2? head?

All i can think of mate.