[Asshat]

Originally Posted by The Rocknrod

Oh I think I see what might be the issue. If you look at a National Pipe Thread Reference Chart you'll see that a 3/4" Pipe thread is a tad over 1", they don't go by the normal nut and bolt references most of us are used to.

Not sure what that has to do with National Pipe Thread sizes.
The water pump connections are cast. Just not sure why they chose a size different than the standard heater hose size. Unless it was intended to match the heater core inlet, which is also larger than 5/8”.
Is there a special hose from the factory that went from the water pump to the heater core?