Welcome.
The 400's all got the hood scoop hood. As a quick check, are some of the other 400 signs there? Is there a 400 emblem on the trunk lid? Do the front grills have chrome "hocky stick" shaped pieces? If it is not air conditioned, are there two metal baffles between the bumper and radiator support? Does the front bumper have a rubber-like Pontiac arrow emblem in the center? If not, it was possibly not a 400 originally.
To be sure, you can get the original build record at PHS.
Jim