Alternator belts - how good does the alternator alignment need to be?

Could not find the alt brackets for my 65 Fury Cop car with a 383 so I am making them. I have having a very hard time getting a good belt alignment.

On this particular setup (Leece Neville), the alternator sits on top of the motor. It is a 2 belt setup with an idler. The belt is a long 7590 and the center to center on the alt pulley to idler pulley is 18 inches and idler to crank pulley is 12 inches.

What would you think the maximum misalignment I can get away with?

Picture of an original setup.