I'm proposing to do just that SS, but could you suggest a good valve - in
particular I'm looking for minimum constriction (my existing 3port valve
has diddy holes for the coolant (heatant?)

I'm using the Myson Powerextra 2 port (I have four of them so far, with more
to follow). They look well made and have a neon to tell you when they are
powered, which is good for diagnosis.

Christian.