No cooling on 1961 18hp Evinrude

Changed the impeller and checked for flow using drill on drive shaft, good head of water, fixed onto main leg making sure water pipe fitted to pump housing, all Ok. Plenty of depth in a water butt about 4 inched below exhaust vent.....Small amount of water from exhaust upon starting then nothing. I've checked the thermostat housing without thermostat and it's quite dry except for a bit of steam. I've blown out where I can with an airline. This is a renovation project and there are signs of ' some ' overheating on the exhaust side of the engine and a ' some ' on the cylinder heads. Could there be a blockage somewhere and is it the engine off and check the usual possibilities or am I missing something ??

Water needs to be well above the exhaust.

The water pipe MUST be installed in the exhaust housing grommet before you install the lower unit.-----Water in your test bucket must be 8" above the impeller.

Still scratching my head on this. Plenty of water above the water pump, water level is about 1" below the exhaust exit and still no water coming out. New pump fitted and tested on just the lower leg with electric drill, plenty of head....I have ran it several times and after about 3 minutes the temp on the top cylinder is around 100c and the lower one is about 50c, the same is with the exhaust cover, top area is high lower area is low. I've even ran it without the inlet cover in place in order to maybe increase inlet flow and I'm running it with no prop in a test tank....would it make a difference if it had a prop on and was in gear so water could be forced into the inlet gauze next to the prop.????