What I've done to bring coolant to the manifold is to modify the oval block off plate at the right/front of the head to provide a hose nipple and then return to a "T" at the heater hoses to make a clean installation. As far as your running on problem--if it hasn't been done already--have a working vacuum retard distributor to aid shut off--more involved but very effective--adapt an electric idle valve (Renault 5/LeCars Webers have them as well as Fiat Webers (I think) and VW Solex carbs. Your stumble from rest and while at speed is not normal--making sure the progressive fuel orifices in the throttle plate area are clear, acceleration pump diaphragm is good and pumping sufficient fuel, throttle linkage set up to provide slower opening on tip-in (relative to the pedal position) are areas I'd look into as well as determining if the jetting (idle, mains, air correction and in extreme cases--the emulsion tubes) and float level are correct for your motor.