The blocks are somewhat different, but the basic Allison V-1710 had a great deal of interchangeability, the basic design did not change much, the bore and stroke stayed the same, as did the rod length. The late engines had more compression and more counterweights on the crank shaft. Some that I saw had bigger cams. It was sort of odd, but very advanced. It had four valves per cylinder, a forged crank, forged pistons, and forged rods. It also had over head cams with roller followers, dual spark plugs, stainless valves, and a pent roof semi hemi combustion chamber. The intake manifold did change over the years, as did the pistons.
I'm not sure what the standard is for "original" as it pertains to aircraft engines. In restored cars, the requirement is for the block, heads, intake manifold, carburetor, exhaust manifolds, distributor, and water pump casting to be original. However, the block is the only piece actually serialized to the VIN on the car. So, as long as the other castings have the correct casting numbers and date codes, it'll pass, even if they are not the original pieces.
