What is a 318 poly wide block?

I believe you are referring to a 318 max wedge.

This answer is wrong. There is no such thing as a "max wedge" 318. The poly or polyspherical head 318 was considered an industrial motor used in early 50's to 1966 full size Dodge Plymouth and Chrysler cars and dodge or Fargo (Canadian) trucks. There is not a big market for these motors due to the popularity of the 413 and 440 max wedge motors used for hot rods in the 60's. I hope this clarifies any questions you had.