The first thing to look for to see if your engine is an original is the valve covers. The screws for the valve cover will be offset on a '58, not straight across from each other.
The next thing is to see what the block casting numbers are near the distributor on the rear drivers side are. You also want to check the build date and decipher the stamp pad on the front of the engine on the passenger side just in front of the valve cover.
THe heads also will have casting numbers and date codes under the valve covers.If there is a twist cap on the valve covers, the engine is most likely a later model. A '58 engine will also have an oil filler/breather cap on a long neck coming from the front of the intake manifold. On the rear of the block, it will also have a breather tube running down and be open near the starter. My '58 also had "X58" heads. That marking is cast on the side of the head near the exhaust ports.
