They are only original when they come from the factory. Putting a later V8 or even a six doesn't make it "somewhat original".

I'd advise putting a 235 in it. It is just as "reliable" as the other engines you mentioned and is period correct.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.