The fist thing I would try is changing or putting the old impellor back in to be sure that you haven't fitted a new but duff one.
Have you replaced the anode and had a good dig around inside the block through the hole and also the raw water
drain tap located parallel but further back almost under the exhaust
? When you take the anode out if you run the engine
pour everywhere showing a good flow not just the block draining down. Alternatively try the hose coming off the water
pump but its close to the fan belt.
The hose that go from the inlet to the gearbox
should be easy enough to check and the pipe that goes through the gearbox
possibly with a coat hanger and mirror.
Finally there is a copper pipe that raw water
runs through the engine that is liable to block up. You can see the exit at the back of the engine under the exhaust
manifold. I haven't been in that far but its another possible problem.