water pump pilot stud size

[ReddingScott]

I am going to order a new 6 blade radiator fan to aid in cooling but the stock fan is so close to the radiator, I can't see the pilot hole size. I see that it can be either 5/8" or 3/4". Can anyone determine for me the correct pilot hole size for the spacer that I am going to need? I have a stock 312 engine. Thanks!

[Hoosier Hurricane]

Stock '57 would be 5/8.  Some trucks had 3/4, but there were no 312 trucks.   John