It has to be water flow . raw water may not be flowing enough . Water pump on engine not pumping enough . Did you check the water pump impellor on the engine there is a cover on the timing cover check if the impellor is worn . I don't know how you are checking your leg pump most guys use the ear muffs on the legs and its not a good way to check your pump as its being feed under pressure when I have heating problem I run the leg in a drum so you get to see how well the pump is working
Find the hose that comes of your leg and connect a garden water hose to it and turn it on . Follow the route of the hoses and remove one hose at a time to see if something is restricting water ( example ; check water flow before manifold and after ) you need to eliminate everything that water goes through. If everything is flowing good then check your raw water pump . Put the leg in a drum leave the hose of the leg start the engine and see how much water the pump is pumping .If all is fine then its your water system on the engine
It can only be the water pump on your engine . you have done every thing else
Spares might be able to help you more as car mechanic hes a boat one
GOOD LUCK
