For 1965, unless you have a build sheet, window sticker, or other credible documentation for the car, there's really no way to definitively confirm that the car came with the 425 HP 396. There are however a few things that you can look for that might point you in the right direction. The most obvious of which are matching numbers - do the engine and VIN numbers match? That will at least tell you if it's the original engine to the car.
There were only 1,838 396's installed in all B-bodies for 1965, so even the 325 HP cars are fairly rare as they were still in the process of phasing out the remaining 409 inventory. The 396 was first introduced on 02/15/65, so the casting date should be after this date for either the 325 or 425 engines. The build date of your car should then be sometime between mid February and the end of the `65 model year run. The 425 HP block casting number for `65 will be 3855962. It should also have " Spec. High Perf." cast next to the number. It will be a 4-bolt block with an "IE" suffix in the engine pad stamp. (All were manual transmision only). The casting number will be on the lower right side of the block next to the freeze plug and in front of the starter motor. There were also no dowel pins used on the timing cover for the 425 HP motor.
Hope this at least helps.
`68 L72 SS 427