We're just guessing here, but it would seem that Chevrolet had bigger bore diameters in mind when it designed the Gen II big-blocks. With a bore spacing of 4.84 inches and a potential maximum safe bore size of 4.625 inches (theoretically, for the big-block design-most 396 blocks can only be bored out to 4.250 inches), the 396's 4.094-inch bore diameter is laughably small. Notice how the deck of this 454 block had to be notched to clear the intake valve? That notch is even bigger on a 396. This is why larger intake valves don't work in a 396. Just like in the cylinder heads, air does not flow well within the confines of this bore size. Finally, the piston size is just strange. In the '60s, pistons for 396/402s were more readily available, but with the trend toward building bigger-displacement engines, your options for pistons are limited.