Welcome to the Team Paul!
The 396 was introduced in 1965 in November. GM's senior board couldn't make up there mind whether they wanted to use the 409's mystic (positive press with songs written about it) and build a 409 Mark IV engine (using the 409's bigger bore and shorter stroke, or go with the torque monster long stroke 396). The managers finally decided to go with the 396 (after building 150 409 versions for testing and evaluation) but because they procrastinated for so long the first six months production of the 396 block had very thick cylinder walls that could be over bored 0.381", which is more than a quarter inch) if you wanted to obtain a 0.060 over 409.
The 425 horsepower 427 was introduced in 1966 in the Corvette (there was a 390 horse version built for the full size B-body cars). The only way to obtain the 425 horse 427 in an Impala was to order it in a special edition car called the "427 SS Impala" (note that is not the same as an Impala SS with a 427 under the hood). It had a very special limited production blistered hood and F-41 suspension with Corvette disc brakes up front, just like you would get if you had ordered a Camaro SS, or a Chevelle SS. Everything on the 427 SS Impala was heavy duty.
Since the 427 didn't exist in 1965 it couldn't have been factory installed in the Impala that you are looking at. The ZL-1 427 you were talking about back in 1962 was a Mark I 409 block that had been stroked out to displace 427 cubes. It was a Mark II engine that was a prototype of the Mark IV 369/427/454 engine in that it used the newer head design with canted valves and a combustion chamber in the head instead of a flat head with he chamber in the piston like the earlier 409.
The Mark III was a still born wider bore centered block similar to what is currently run in the 500 cube Pro Stock Classes now. Chevy was looking at the costs of converting from the Mark I-Mark IV's 4.84-inch bore center to a bigger 5.00 inch center between the cylinder bores. This would have allowed for future growth on BBC displacements with a reasonable length stroke that would rev quickly (at the time Chevy was all about over square engines such as the 409, the 327, and the 427 and the 502; that all had a bore measured in inches that had a larger number than their stroke measurement).
Newer engines such as the 305 SBC and the LS-x engines are all under square where the bore is smaller than the stroke is long the same way Pontiac and Oldsmobile engines where built back in the day. Because this new BBC engine would cost a LOT in new tooling at the Tonawanda, NY foundry. This is why GM cancelled the project; though there where over five hundred blocks cast for engineering evaluation.
So the 348 and 409 are Mark I engines; the ZL-1 427 "Mystery engine" was a Mark II, the Mark III was still born leaving the 1965-1990 Mark IV series as what most are familiar with when you say big block Chevy. The newer Gen V and Gen VI blocks have a one piece rear main seal, and all Gen V, Gen VI blocks are four bolt main blocks with the Gen VI using a hydraulic cam. The Gen V can be distinguished from the Gen VI by a lack of a mechanical fuel pump boss on the front of the block (since they were all EFI, they didn't need a fuel pump driven off the cam and since they where installed in front of an automatic there is no clutch ball boss to use a mechanical clutch linkage)
Last edited by Big Dave; 02-17-2016 at 08:42 AM.
Reason: Inacurate date given