Q Do all true 67/68 400 firebirds have 3F on their dataplates?

A Just 67.

Q Why do some 67 400 firebirds have flat hoods with 400 emblems on it? Thought all 400 cars came with the scooped hoods.

A Somebody put the 400 emblems on the flat hood.

Q Why do some 400 cars have the 400 emblem on the trunk and others do not?

A Replaced the truck lid.

Q How can you tell if a 400 car is NOM. I know at some point in 68, the vin is stamped on the block...other then that, is it only a guess based on the car having a pre-car build date stamped engine (ex. engine built or date stamped in 2/67 and car built 4/67?

A You answered that one yourself.

Q Any other ways to tell if a car is NOM or a true 400. I know a 400 car will have the extended grill moulding and emblem on the front bumper..anything else.

A The only 'fool-proof' way to tell if a car is anything, including a 400 car, is with PHS.