From my research, and some first hand info from relatives,
I have a couple thoughts.
1. In the '70 model year there were at least 5 .....400 cuin. Engines Built by Pontiac. SAME BLOCK casting.
2. Lots of #12 and #13 heads were readily available. And show up close to the block dates in most cases.
3. Intakes, plentiful and a ton built in Oct, Nov for the 70 model year, shared with the 455 as well, show up pretty close to blocks casting.
4. This leaves the "D" Port RA Exaust manifolds.
This is where I know there was supply issues. Dates on Originals have been real close to assy, and far from block casting in many cases.
If there were plenty, the 455HO Would have got them on the GTO more than likely.
So I attribute many, NOT ALL, date " oddities" on RA engines to Exhaust manifold availability.
Some blocks were " Training Blocks" as well as poor casting blocks. Lots of waste.
Most all 350's ( same crank) and 400's of 70 that are Not RA SEEM to be closer in dates across the cast dated components.
In the late 70's we had big delays in getting these same manifolds due to cracking and leaks demand, and some OTC demand.
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"The Future Belongs to those who are STILL Willing to get their Hands Dirty" .. my Grandfather
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