I'm sure you realize that's a Chrysler poly, either 301 or 331 or (if I remember correctly) 354. Chrysler block, Chrysler exclusive poly heads. They went into the low line Chrysler Windsor/Saratoga. Nothing interchangeable with the 318 poly.
There also was a Dodge poly, in 241, 259/260, 270, 315 and 325 cubes (the latter two being raised deck). Confusingly, some early Plymouth V8's (all 55's and some 56's) ran Dodge poly motors. Also 57 DeSoto Firesweeps. Likewise, virtually nothing interchangeable with the 318 poly.
The first true Plymouth designed V8 was the 277 poly, introduced in 56. That's what's in my 56 Belvedere. It looks just like a 318. There were also 301, 303, 313 and 326 versions. 303 and 313 were Canadian (303 also used in 56 Fury), 326 was 59 Dodge only. Here's my 277, stock 200 hp power pack (4 barrel and dual exhaust) except for a 500 Edelbrock carb replacing the original WCFB, and a Mallory Unilite ignition:
View attachment 1716011987