The 5th letter of the VIN denotes the engine the car was built with. A "Q" for the 428" and a "Z" for a 390.
Below is the info I found to confirm my car has the 428 engine. The number on the lower right bank, (have to get under the car to see it if the engine is not out of the car) stamped during casting (and difficult to read!) will start with a C6.
Once you get the stamping number, go to the web site at the bottom of the info, and scroll down to the C6 listings, you should be able to find the data for your engine.
(Note) the info below is from the engine in my car, and it starts with C7, as it is not the original 428 engine, but was replaced with an engine block from 1967 Mustang, as I was told when the car was purchased!
390/428 engine :
Block- casting date- 7 (C or G) 31 1967, Mar or July 31st
Number on lower left bank - 24 D1F
Number on lower right bank- C7ME-A ('67-70 Police Interceptor 428 4V Cobra Jet)
Intake Manifold: 6E20 - 1966 May 20 (Casting date?)
Also on runner:
C6AE, ('66 428 cu) 425-G
Engine info web-site:
http://www.erareplicas.com/427man/engine/partnums.htm