Are All 340 Blocks the Same?

Like 66 said - any iron casting is made by pouring molten iron into a mold. The mold is full of cores. Cores are basically where the iron will not be - so the cylinders each have a core, the crankcase has a core, etc. If at some point in the casting process one or more cores move before or during the molten iron pour, the resulting part will "exhibit core shift". In blocks you see it when it's sonic tested as thinner cylinder walls and an odd seam where the top and bottom cores meet right under the distributor in the valley area, or a larger "ear" of iron on one side of a lifter bore (so the hole looks like it was bores off center). On heads it's port alingment...
Core shift in blocks is what makes all the stories about "thick" and "cold weather" mopar blocks the stuff of legend. Mopars have a lot of core shift - especailly the 340s and I'll expand on Mike's and say the 72/73 340s seem to be the least shifted. At least the ones I've tested.