How do I know if I have a poly 318, wide block 318 or small block 318?

I am looking for headers for my 1966 Plymouth fury III it has a 318 in it with the stock cast iron exhaust manifolds on it. My bolt pattern is like this ": : : :" I cannot find any headers that will fit on my 318. I'm starting to think my 318 may be a "Poly 318" Can I please get some advise, Thank you! -Kevin