So I should not list the car as #'s matching then, even though it looks like a #'s engine. The partial VIN on the engine does match the VIN of the car.
I'm looking at my PHS docs, where they highlight what my car came with, and it clearly shows a 400 4 bbl YT L78 engine with a PX M40 transmission. There is no YS on this powertrain chart at all for 1972. For the rear end, it states 3.55 gears for non AC, and 3.23 with AC. My car is an AC car.
Hmmm, very interesting. I am using the GTO
Association of America ID Guide and it does not list the YT engine code BUT, when I go to the Wallace Pontiac engine codes, I do see a YT code for the 250HP 400: Pontiac Engine Codes
So next I pull up my Pontiac engine book by Pete McCarthy and here is what I find- Both engines are identical 250HP/automatics and are the only engines used in the A, B, F, & G model lines - or all body styles. Other engine codes are specific to just a model line or a couple of model lines. So the engine seems to be a common engine for 1972. Again, no YT code for 1971.
The next thing you need to do to answer a few questions is to look at the back of the block near the distributor. You will find the year (typically a 2 digit casting number) and the date code as to when the block was cast. This should be close to your build date as should the date codes on the 7K3 heads.
I can't help much on the transmission other than what I have already provided.
I don't think all the plants went on strike, just the Norwood, Ohio plant which made the Firebird line. I don't think it affected the A-body Lemans. I read that to get as many as they could out the door after the strike ended and before the 1973 year models came out, they cut corners and did not undercoat them and just painted the exterior. I looked at a 1972 Firebird Formula 400 that a guy wanted to sell cheap, but was rusted very badly from the inside out - door jams were blowing out, so it made sense that they just threw these cars together. It really needed a complete body to save it. https://www.nytimes.com/1972/09/28/a...-174-days.html
I would contact PHS noting your build sheet and what you have found with your car. He would be better qualified to give you a definitive answer as he has seen build sheets that don't seem to match for one reason or another.
It seems odd that the engine appears to match the car, yet has the wrong letter code. Possibly the block was mis-stamped or the Pontiac paper is wrong, especially if there really is no difference in the engines.
So I would wait on PHS before advertising the car as original/non-original numbers. Also check the block dates.