If you can't get a wrench on the shaft between the pump and motor, then you can remove the end of the motor, and there should be some flats on that end of the shaft to get a cresent on.
Goulds has a service manual on their site. On page 68, it says:
9. A.O. Smith Motors – Remove motor end
cover. Insert 7⁄16" open end wrench under
switch mechanism or behind overload
protector onto flats on motor shaft.
While holding the shaft against rotating,
turn the impeller counterclockwise. The
impeller should turn completely off the
shaft in this manner.