Check that the O-rings for the small tube between water pump cover and manifold, fit properly.
I have seen a few that were too thin, so only had a marginal seal.
Also the O-ring between manifold and head can cause problems.
Again the ones supplied sometimes are not correct.
And fit the manifold without the carburettors, as it is much easier to handle without them.
A few pictures and some more info in my blog:
https://tr7beans.blogspot.com/2010/09/water-pump-here-we-go-again.html