I am not too sure if RA IV heads were date coded but if they are, the build date on cowl and the date of heads, if close, could assert that the block was replaced, and not the entire engine. What you would really look at is..

Date code of heads
Date code and casting code of intake
Date and stamping of carb
correct RA induction?
correct exhaust manifold casting/dates

If all the dates are close and within parameters set by GM/line production, and the block is a SR, likely just the block replaced. This is good news.

If the date codes are all scattered, likely the owners in past lost the entire engine and pieced it together. Bad news. A true GTO buyer will be able to spot the issues if the block was collected and transplanted.

Again, I am not a GTO expert but am familiar with alot of the Pontiac practices.

I am unsure of when the RA IV coded GTO's were produced. Some models/options were produced throught the model year. A good example, and as an example (my dates and production numbers will be off) is the 67 RA I Firebird, 65-70 produced..None existed until the late June or "F" code, into July "G" date code. One can assert that a 97 head with a date code earlier than an approximate cast date of F287 or a cowl tag earlier than F4 is not a RA I bird.

Again this is an example...

You will be able, with a little research, ascertain the true validity of all aspects of the engine.