If the numbers you find on the block are cast, that would be casting date instead of manufacturing date, which can preceed the manufacturing/machining date by up to 6 months. As the first 400s were for the 1972 model year, I suspect that's what you have.
To me, key points would be if it's the normal 2bbl, 4bbl, or HO 4bbl version. Up until the end of 400 and 440 production, the 4bbl police motors were basically '68 Road Runner or GTX motors with lower compression (except for the bore difference between a 383 and 400). Windage trays, the chrome rings w/moly inserts on the compression rings, better bearings, better timing chains, the last variation of the "Road Runner" camshaft, hd valve springs, and similar high performance items were there to the end on those HO police-spec motors. All 400s were cast cranks, EXCEPT if they went into a B-body midsize car with a 4-speed trans, which got them steel cranks instead (verified by the 1974 Chrysler parts book).
All 400s were just as good of a motor as their previous 383 cousins, just that they hit at a time when everything had to have 87 octane unleaded fuel and the resultant 8.2 compression ratio. The '72s did not have induction hardened valve seats, but then low-lead fuel was still available then, so Chrysler recommended every 3rd tank be of some type of leaded fuel. '73s did get the induction hardened valve seats. The added displacement tended to mask the loss of power from the lower compression ratio, but also at about 1mpg (by my best figuring) loss in ultimate fuel economy.
The cylinder heads that were revised for the 400s era motors were originally reputed to be "smog heads", but later research and investigation has determined they flow just as well as the earlier heads did, just with a little different port configuration. In other words, they make the same power as the earlier heads on the same engine, so they are not a flaky as originally suspected. Chrysler kept the same 2.08/1.75 valve sizes too as they always wanted to have good heads to make their motors "better air pumps".
If you put a small chamber head (73.5cc blueprint chamber volume) on a 400, it'll put the compression ratio up about 1 point and get some extra horsepower.
The other side of things is that you can use a 400 to make a low deck stroker motor, using a 440 stroke crank with its mains cut down to 383/400 size. Some of the magazines touted that conversion several years ago.
The 400HO motors in the Charger SE, Cordobas, Small Fury, and Coronets were rated at about 245 horsepower and were spec'd without catalytic converters for the 1975 and 1976 model years (complete with a "Non-Catalayst" decal on the driver's door). These motors also came with small, high-stall torque converters too (some with stall specs up to 2800 rpm! for the similar 440s). These were the police-spec, factory high performance motors I mentioned previously, that were available if you knew the correct option code to order in regular cars.
Many people might shy away from them for performance use to the cast crank they came with, but it's documented that these cast cranks were heavier-duty versions and had no strength issues for most high performance, naturally aspirated applications. Of course, a 383 steel crank would drop right in if you did a balance job with the rebuild, if that sort of thing really matters.
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