If I put a 153 tooth flexplate on my sbc how much difference will it make in how it starts? It will want to spin the engine faster so assume crank over harder. Is it noticeable and is it hard on the starter? I'm guessing 9% which is the difference from 168 to 153.
I need to do this to get my headers to clear the starter.

In my experience, it will be fine. I have done the same with some packages.