Thomas,
There should be a small plaque on the passenger side (left side when facing the car in case right hand driver) under the lip for the hood. Should be a plaque with round ends that states something like “when ordering parts for this vehicle” and a BN1 “number”. If it’s a matching car the engine will have the same number with BN1L or something like that, this plaque is riveted to the side of the block below and to the left of the distributor.
Good luck often these tag on the engines are gone but hopefully not yours. Chassis ones are typically there. Also a few other places to look, on the rear vertical lip on one side of the hood usually below a few coats of paint and on the truck mounting tab where the trunk brace connects to it. It’s a Circular tab. All of these places would match on an ideal car, I suppose….
Good luck,
Michael.