HP500 has never been a "hand assembled" engine. Rotating assembly is assembled by GM hands and never touched again at Merc.
525EFI actually IS assembled at Merc.
And WHY are you comparing 496's with 502's? They are NOT EVEN CLOSE to the same motor. A 496 is a 454 bore longstroker motor. A 502 is a big bore siamese std stroke motor.