Yep, that's the original 327/250hp engine. HC is 250hp 327 with powerglide.
The protectoplate would be in the back of the owners protection plan and new vehicle warranty manual, which was kept with the owners manual in the glove box. It was used for warranty work and has the car's drivetrain codes on it. Does the bottom left of the inspection sheet say MLD or WLD?
I noticed on the inspection sheet that the boxes with the RPO codes have little letters at the bottom of each, C, O and P. Arlington produced Chevrolets, Oldsmobiles and Pontiacs so it looks like they put the letters in those boxes to show which RPO applied to which of the makes. Z18 "Caprice Option" is shown with only a C, which makes sense.