I just acquired a '65 327 250HP engine. The casting number is 3858174. It has a very late casting date G 19 5. It has a stamped assembly date of T0729EE. The intake manifold has a E 24 5 casting date. The alternator is dated 7D5. The heads are both July dated as well.
The engine came out of an all original '65 Malibu and I believe it to be the original engine.
Can someone shed some light on the build out period between production years. What is the approximate latest production date for '65 Chevelles?
The 3858174 casting number is a somewhat confusing casting in that it seems to show up in few, but various cars in various years. Alan Colvin's book, Chevrolet by the Numbers, refers to a 3858174 block as a "mystery" block.
Any information on this '65 engine would be appreciated.

The latest production date would be in late July. Since the model year changes in September, the tooling for the next model year would be in the process of being changed in August sometime. My 70 is a very early production of 08C. Here is some decoding on that casting number though:
64-66 327 Passenger car, chevelle, camaro (small journal)
64-67 327 truck (small journal)
66 327 corvette (300, 350 HP small journal)
67 350 camaro

I have a "174" 327 block, With a 67 cast date. It was L-76 engine out of a 67 Corvette. I was told it was not the orig engine for the Vette. It did have Std 11.1 stock pistons, 2.02 heads, 7 Qt oil pan with windage tray and 8" balancer, Same intake & Carb as the L-79, A stock solid lifter Cam. All parts were Std and had Chevy part #'s. It Has always been a "mystery" engine to me. The oil pan & windage tray would not fit under my 64, I had to use a pan from a Chevelle.

I'm not sure what is so mysterious about the block. It is a 327 small journal 2-bolt block made from 64-67. The 870 block was more common in those years but hardly the only one used.

Looks like you've got one from the end of the model year run as already mentioned.

My guess was on the 67 block that it was a warranty replacement block. The third owner of the Vette had this engine pulled and installed a counterfit "Matching #" engine. So if you ever see a matching # white 67 roadster for sale. Look it over good. Every time I see Matching#, I think of the 67. This was 15 years ago, It could have changed hands many times for"Big Bucks". IMOP the engine I got was more of a Orig engine than the one that ended up in the car.