this is the best way, if the engine still is in the car. Otherwise, there is no sure way to tell with any markings. The same casting numbers were used for 390 and 428s. 428s are very rare, and that alone would lead to a 390 in the car. The 428 cars were very special!! To tell the difference without any disassembly, you need to remove a spark plug. You then want to set a pencil or similar object on the piston and slowly rotate the engine. This will cause the piston and pencil to move up and down. The 428 have approx a 4" stroke whereas the 390 is closer to 3.75". No other markings will tell you for sure, trust me. I have a 428CJ Mustang and a 428 in my 69F100. I also have another 428CJ MUstang in my shop right now. They all have different markings, I have played this marking game for over 20 years with FEs, its fun.