Since you are positive about the JAD stamping, then it has to be a '51 235 PG car engine, unless it was originally stamped in error, or restamped by someone.
The block casting number is somewhere on the right (passenger) side of the block, usually near or possibly behind the starter. Later blocks had them up front by the fuel pump. The block date code is usually by the starter too, possibly after the word CON. The head date code is probably on the right side of the head, around the area between the 2nd and 3rd spark plugs. There may be a bunch of numbers cast there. Paint, grease, and other crud could be concealing the numbers. Date codes are a letter followed by two or three numbers.
Just out of curiosity, what is your truck serial number (VIN). You said it suggests a '53 truck.
