67 had the 3F but there is nothing on the '68 data plate which identifies engine.

400s came with scoop hoods, grill chrome "hocky stick", emblems on the hood and trunk, dual exhausts (not totally unique to 400), and a few other options shared with AC equipped cars like baffles and fillers enclosing the radiator area.

Those items may provide a clue to identity but in 37-38 years lots of stuff got replaced, added, switched or removed so that only a PHS documentation can tell you for sure.

You can use a number of engine markings to help identify the car's engine. See: http://www.teufert.net/identify/identify.htm