1968 Plymouth Valiant First Gen HEMI
I love your ambition. The Stage I Hemi is a MFer to put in the 67-76 A body platform. As you are likely aware the early Hemi is an incredibly wide platform and requires being centered in the engine bay to clear the inner fenders / OEM shock mounts. Might not be too bad if it were not for having to have some kind of exhaust manifolds / headers.
pictured below is a set of 2"od headers built for a customer in Michigan doing a similar build except with a A body 727.
Hot rodders are a special breed. We'll put a helicopter engine in a submarine if challenged.