The 331 Poly is, as you said, the economy version of the Hemi. The block is the same, except for some minor machining in the pushrod holes. The heads are simplified for cheaper manufacture, with only a single rocker shaft instead of the Hemi's two. The intake is the same as the Hemi, the exhaust more like a conventional OHV engine. The 318 is a different case, there never was a Hemi version of it.
Find a set of 331 Hemi heads, they'll fit your engine. Your manifolds work, you need the Hemi exhaust pushrods though, different length.
As for it fitting your Pilgrim, well, that depends on what your definition of "fitting" is. It'll go in there, but it ain't easy.The steering box is in the way of any V8, as I recall.