Look at the data plate on the firewall near the distributor. The serial number will start with the letter C,D,E, or F. The second character will be a 7. Most likely you have a D code. 312 C.I., 245 horsepower, single
four barrel carburetor. As an asid there are a very few D code cars that are referred to as DF cars. They are the early supercharger cars and have slightly different fuel filter than F code cars and a few other visible differences. A DF car requires documentation for proof of status as a factory supercharged car. Porthole has nothing to do with the engine installation. Porthole tops became optional in 56 and were relatively standard on 57s. However there are some non-porthole tops that look like the 55 or 56 top without a porthole but they have a different badge.
good luck and buy a
shop manual.
If you can't afford a
shop manual you can't afford the car.