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 does
water 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.
Pete