It would have to be a late 56 I think. Because IIRC that was where I found out about the J2 shim head gaskets. The 371 came out in that time frame.
Ya gotta love Olds Rockets !! When I brought the El Caballo back to life, I used a '58 371. It originally had a 303 which was out to 380, so I figured I was close enough. Ak Miller and Ed Iskenderian reground the original cam, and Al Blazic of Oakland did the balancing. I used a Weiand 2x4 manifold with 4 48's mounted in pairs, back to back. Not a lot of mods, but it ran like stink. Ak and I ran it in Mexico and at Laguna Seca together, and I raced it for 10 years. Most fun drive ever !!!!!