[unruhjonny]

if you don't mind my chiming in;
generally with respect to model year engines casting dates "early" is a term used for cars built within the first month or so of the model year commencing;
Any early 1970 model year car would have been built in August, or September.

You car being an April build car (and the very first "1970+" Firebirds being built in late January) puts it about at the half way point of the 1970 Firebird model year run (I have simplified things there).
I would guess that your car would probably have an engine with a March casting date - but there are many examples of both Firebirds and GTO's which show a much wider than expected spread between build date and casting dates on RAIII engines than that of more "bread and butter" engines - so maybe a month or so older might work too.

I would think that January or older casting dates would for sure be too far in advance to your car's build date - but honeslty a partial vin/eun would be a really good way of nailing down when a given engine was installed in a vehicle.

As a point of reference (because I am trying to help after all), I can cite my RAIII.
I believe I was told by a member here, that based on the partial vin on my YZ engine, it would have been installed in a first week of February car - my engine had the following:
block: January 22nd
intake: January 21st
heads: Sept 2nd & 10th (69)
distributor: July 10th (69)

What seems to muddy up things is that parts were made in batches, and it often seems that the block and intake are closer in their casting dates than the other parts - which may explain why my engine (as acquired in 1995, and not modified as far back as I have been able to find, which was 1985) had heads and a distributor that so far predated the block/intake.

I hope this helps.