I have a '54 and have done the same thing. It's a tight fit, but it works. I installed the water pump and fan and then slid the radiator in place. There's only about 1/4 in. of clearance. I also had to shave some of the blade off so it wouldn't hit the harmonic balancer. Maybe for some reason, the pump you got is not correct. I'll check the length of mine when I go to work tomorrow and give you a spec.
