Brian,
For the most part, other than the service block, the 390's are pretty much the same. They will differ more in cylinder heads and intake design than in the block.
I'm not sure how many different heads there are, but taking off a valve cover and looking for a part# is the way to start. There have been many threads about casting numbers for FE parts, but the simplest way to remember is:
C-7 xxxx is this: C= 1960 decade. 7= 1967.
If you buy parts from anyone, have them write you a receipt for parts from 1967 or earlier. It is highly unlikely your motor vehicles department is going to ask you to tear down the engine to somehow prove it's age. It is better if the receipt is from a dealer or business rather than from a private party. Less likely to make them ask questions.
With the aluminum heads, the age gets tossed out the window anyway. DMV people are highly unlikey to know about that aspect and frankly, they probably really don't care. All they want is documentation. Here in Oregon, all I did was take my receipts in and show the counter man at DMV. Once he looked at them it was a done deal and I left with a 1966 title in my hands and my receipts went back in my pocket. I don't know if Nevada is like that or not, but why wouldn't they be?
If you're worried, go to a small town DMV, instead of one in Vegas. They are usually less picky.....at least that's true up here.
By the way don't let this correct vintage thing bother your conscience. Any FE is a smog maker, so no matter what you put together, it's going to pollute the air.......and that's exactly what Cobras are supposed to do......be noisey and smell bad (good?) at the end of the pipes.
Al