the 318 is an A ENGINE poly,,not to be confused with the 318 LA motor,,but a few internals are the same,,as far as i know the 318 A poly does not fall under the catogory of the early poly motors,,the 318 A poly came in around 1959 till 1966, that block is more like a LA motor

the 1953 thru 1956 poly motors are differnt and are the base for the same year hemis,,,

dodge polys share the same block crank rods as the dodge poly of those years these can be converted to a HEMI by switching to hemi heads and pistons,every thing else is the same,.

same for the desoto poly/hemis

and the chrysler 1953 thru 1957 hemi/poly.,.

none of the parts interchange from dodge to desoto or to chrysler they are all seperate familys,,,thats why production of these great motors stopped,,nothing was the same and too costly at the time to keep producing 3 competely differnt engine lines,,,

i dont know how many if any of the same vintage plymouth poly motors have any thing in common with the dodge polys,,i know some of the cubic inch are not the same as dodge,,,

some distributors and oil pumps can be made to intrerchange