I used all quick silver parts I had that manifold off but not out it was laying all kind of ways in there I just read that a ball valve could be stuck in the bottom and letting all rhe water go to manifold and not hold it back on the engine wich would make sence manifold is cold t Stat is hot
there are check balls in a T on the top of thermostat housing on some mercruisers. This is for the warm manifold style where the manifold and elbow are fed by separate cooling lines. the one that goes to elbow has constant cold water flow regardless of thermostat being open or closed. If it send too much incoming cooling water to elbow it starves engine of cooling water.
At this point double check the gaskets between manifold and elbow, thermostat, verify water flow from outdrive at thermostat. I think the test merc has is take the hose off from outdrive - it will be the 1.25" or so hose on RH side of thermostat housing that lays on top of intake manifold. it should supply 3-4 qts in 15 seconds/3-4 gal per minute or so.
Was the old impeller missing any pieces?