Call or email Fast Lane (sponsor of this web site) for water pump info and purchase for your 300. I just rebuilt a '77 300D euro engine and dropped into my US 300D - swapped water pumps since the one in my old engine was only a few months old. The water pumps were the same.
Replacing a water pump has been covered lots of time. Search the tech or diesel forum for info.
America: Land of the Free!
1977 300D: 300,000+ miles
American Honda: Factory Trained Technician & Honor Grad.
Technical Advisor to Am. Honda;
Supervisor of Maintenance largest tree care co. in US for offices in Tex.