Welcome to the team Chris!
The numbers you gave will match the last six numbers of the vehicles VIN. There is another set of numbers stamped next to that number that will end in two or three letters. Those numbers tell where it was built, when and the original application.
If you post that information I can answer what it came out of and when it was built. There are nine different combinations of carburetor and transmission used in 1966 in a B-body car depending upon police, taxi or passenger car use, though all of the 396 engines in the full size car where called out by the same RPO code of L35 for a 325 horsepower 396. All of the application codes start with the letter I so if you see only one letter say an IB or an IN the I should be an I and not a 1 though frequently since they were hand stamped these two marking dies were interchanged.
Possible codes are: IAH, IAR, IB, IGH, IGR, IN, IVH, IVR. The difference between the two or three letters is caused by the factory switching over to three letters to avoid repeating previous combinations. At this time Tonawanda hadn't cast as many engines as the Flint foundry had so it was still using a two letter code. Your date code is for the first week of February of either 1966, or '76 (or '86, 'or '96) block. The difference is obvious if you can see the rear cam tunnel of the block as in 1976 there was a groove cast into the block to distribute oil to the top end, in 1966 the groove was machined into the back most journal of the cam shaft. That expensive operation was dropped in late 1966 to save money in machining.
By high performance I am assuming your block has four bolts holding on the main caps. That and the extra hole machined above the oil filter for use with an oil cooler in Corvette applications make it a high performance block, other wise it is either a truck block (also has four bolt mains) but is cast out of higher nickel content nodular iron and is taller than a standard passenger car block. This is visually identified by looking for an extra half inch (0.400" to be exact) of block above the top of the water pump. A truck block is also machined for an oil cooler. If it is a two bolt block you have the last the three words cast into the side of the block at the foundry which is Pass (funny how everyone assumes it must be a HiPerf if they see that word anywhere on a block when it also says Pass for passenger car).
The Pass and the HiPerf block are both cast from the same molds. The only difference between the blocks is the machining as a HiPerf gets an extra hole drilled for an oil cooler return line and the two extra holes for the two additional main cap bolts. The main caps are cast in different molds as they shape is different, but the machining of the block main cap registers is the same. There were no high performance big blocks ever installed in an Impala except for the 427 SS made in 1967-'69 which had either a 390 horse two bolt block or a 425 horse four bolt block. All of the 396 engines used two bolt Pass blocks to get either a 325 horse or a 350 horse engine (used different flat tappet hydraulic cams to gain the extra power).
Because of the added height they must use a different mold for a truck block So I have no idea why the foundry includes that word on blocks going into a car. The added deck height is to accommodate the fourth ring on the piston that truck blocks have to seal an engine that spends it's life at near WOT for a hundred thousand miles or more.