For a small block, I believe it's not dimpled, but just not squared off so it has the impressions moving inwards towards the front.
For a big block, I think it actually has dimples.
I did a quick search at Summit (and not commenting on the quality of any of the below, just the shape) and I believe these are the right fit pans:
Small block:
Free Shipping - ATP Engine Oil Pans with qualifying orders of $99. Shop Oil Pans at Summit Racing.
www.summitracing.com
Free Shipping - Dorman Oil Pans with qualifying orders of $99. Shop Oil Pans at Summit Racing.
www.summitracing.com
Big Block:
Free Shipping - Summit Racing™ Stock Replacement Oil Pans with qualifying orders of $99. Shop Oil Pans at Summit Racing.
www.summitracing.com
Free Shipping - Mr. Gasket Oil Pans with qualifying orders of $99. Shop Oil Pans at Summit Racing.
www.summitracing.com
BTW - here is an example of a small block pan that is squared off and will most likely hit the steering. It is unfortunate they say this will work, as it most likely will not.
Free Shipping - Mr. Gasket Chrome Plated Oil Pans with qualifying orders of $99. Shop Oil Pans at Summit Racing.
www.summitracing.com