Ati balancer

I have an ati on my 470 but haven't got around to the pulleys yet. I have to either build or buy a .250 spacer. I'm also using a engine plate for my dart.
So the need for your spacer is to compensate for the motor plate? The only belt on my setup is for a alternator. I have had belt issues in the past and finally have the alignment good would just hate to mess it up. It's a gm style power master mounted on the left side(driver) of the engine.