Fisher did not care one bit which engine went into a car, everything engine related was installed by Chevrolet. Fisher added the parts some of the cars needed. If a car needed a 3 speed, or 4 speed transmission, fisher had to cut the hole for it, if the car was an RS, fisher had to attach the right rear tail panel and punch the correct holes for wiring and trim. As far as the 4'so, 4P literally means it got the engine reinforcement plate under the rear seat on the passenger side. 4L is engine reinforcement with Z28 stripes on the trunk panel, 4N, and 4K is an engine reinforcement and black tail panel. There is lots of discussion on why the 4K exists, but there either was supposed to be some difference between the L78 and L35 that was never implemented, such as two traction bars, or there actually is some difference in the structure of the bodies, that no one has been able to find yet. 4F is the remote mirror, which required a different hole drilled in the door to pass the cable thru. Nothing in the codes specify a specific engine. All small blocks are the same size, as are the big blocks so there would be no physical difference needed to be incorporated into the body to support one engine over the other as far as fisher was concerned.
This car could also have a fisher coding error, instead of Chevy putting the wrong engine into a non SS, in that they did not install the SS components into a car ordered that way, and chevy added all of their SS related hardware, per the broadcast sheets. Things did go awry every so often in the plant.